I'm not great at electrical so I need some advice. 69 Roadrunner, new engine harness 2 yrs ago, last year the dash harness melted and caught fire at the fuse box, had a lot of crappie splices, chafes but was running fine.

installed new dash harness this week, everything seemed to be working fine, had the car running a bit yesterday then hooked up my tach and accessory gauges to new fuse box today. again all seemed fine with the car not running, had key on for a bit and gauges lit up. cleaned up and started car for maybe 10 seconds and fusible link at the bulkhead went up in smoke. gotta get a new one, and I unhooked the gauges I hooked up today. So do i need to figure out if something is drawing to much in the car? or can a bad voltage regulator say put out to much juice and fry it?

Maybe a frayed hot wire?

It's all new I'm afraid it shouldn't be a frayed

yes, dead short to ground somewhere

Check these out:

Make sure harness bulkhead is seated, not loose, ammeter lugs aren't loose, switch connector, short in the alternator.

A dead or faulty shorted cell in a battery with a reman alternator that puts out 55-60 amps vs the stock 37 will do this. It is smart to run a 6 gauge wire from the alternator stud to the battery cable, with a fuse or 10 gauge fuse link, if you have a high output alternator....most remans just default to a 60 amp, couple that with an old battery and poof. So yes, regulator could fail, and full field a high output alternator into a low charge, or bad battery and fry that link in 10 seconds, with stock wiring that has nothing wrong with it. Fuse link is the safety valve.

thanks for the info guys, I am getting a new link and will update when I on HD it, either that tach has a short, or the battery was low cranking this time, it has been sitting a year and had a charger on it when cranking as it had run down after initially charging it over might, which I attributed to having the dome lights etc on while working in installing stuff

Get a factory wiring diagram for your car and make sure the terminals are in their proper cavity. Years ago on my 73 Duster, I found some were not where they were supposed to be on both sides on the bulkhead unit. I switched them out to make them correct. Maybe one or more of yours on the new unit is where its supposed to be but the opposite end is not.

You're playing with fire here.
There is a short to ground.
The only reason a good fusible link burns quick is when too much current is flowing through it.

A dead battery and a high charging rate should not melt it. At idle, even a 60 amp alternator is only going to supply 30 amps or so.

Read this.
http://www.heritech.org/cuda/Charge2.html

You can use the '67 schematic for general guidance. '69 is going to have different connector cavities, a few different colors, and some other differences. But essentially the flow paths are the same.

For trouble shooting, you can connect a blinker modual as a circuit breaker instead of wasting fusible links and also risking other components.
Another method, which could be used in conjunction with the circuit breaker, is to wire a couple of leads on a light bulb. The wattage of the lamp will restrict the current and you'll see there is a problem.
Start with the battery side. key off, doors closed, lamp should be off. If not look for the short ground in the main circuit.

or can a bad voltage regulator say put out to much juice and fry it?

Not really. The regulator is the control valve for the letting power flow through the rotor.
If you have an original type, under the cover you'll see a set of points and some thin wires. The wires are fusible links.

If you have voltmeter or a one of the better battery chargers, check battery voltage when the charging is complete.
Some chargers will show charging current. Pretty much the same as the one side of the ammeter in your car.
It may draw a lot of current initially, but for long deep discharge, that needs to be reigned back for a charge at low current.

if you have doubts about the battery, bring it somewhere that can put it under a load test.

thank you, I hate replying on my phone cuz and make replies super short because of. i am not just hooking up a new link with my fingers crossed...I have the service manual and diagram for the car. I am suspecting the Tach I hooked up has a short in it, as when I first fired up the car with the new dash harness in place it was fine, this happened after hooking in the old tach which my son(this is now my sons car I am still working on it for him though) said would stop working in n the past intermittently.

I will use your suggestion for trouble shooting with a light bulb etc to find the problem so I don't burn up more links, thx!

All it takes is one wire accidently scrunched down or rubbing against the body metal.

Service manual is best.
'69 B Body I'm not that familiar with.
Schematically, it will be almost the same as any late 60s A or B body.
There was definately some change in the connections, and I think the tach wire may have been run through a grommet in the firewall?
Could do a resistance check of the coil itself. Primary connections should be open to ground.

Not sure if you looked through the link above.
It should be helpful.

But for a general scheme of the main circuit (power feeds and everything connected to them) this will be what you want to make up or alter the details for your car.
First one doesn't include the start circuits. If there's no probelm in them, its easier to leave them off the schematic.
Key in RUN, both Ignition 1 (Run) and Accessory feeds get power from the switch. Accessory includes power to flashers, windshield wipers, and two or three circuits with fuses.

When my fusible link blew, it was a short in the alternator that caused it. I reached down and the stud on the alt that feeds the battery was loose internally, typical rebuild! I replaced the alternator and the link and no more issues. If you have a rebuilt, that might be the problem.

Actually, had EXACTLY same issue with "Rebuilt" Alternator also !! . . . was on a trip and good thing I had my "old" alternator in the trunk, and tools, and a bunch of wire . . . got it fixed up on side of road and no issue after that !

Ya I am thinking if it was your tach wire job, and you used normal 18 gauge wire, that tach wire would smoke and the fuse link would be fine. Until you find the problem you can get a cheap fuse link, or wire in a cicuit breaker from napa so you dont keep blowing the factory looking ones. I use a buss automotive circuit breaker with wires soldered on it when troubleshooting, if they still sell them.