I noticed that my Headlights dim and brighten (as does the dash light) when my blinkers are on? I figured that someone else has seen this before and might point me in the right direction.
Car is 1970 Plymouth Cuda.

your battery may be nearing the end and/or your charging system is weak.

Guessing your only noticing this as your car is idling at a stop right?

check out this sight for a ton of info on upgrading your old electrical system.

www.madelectrical.com

You might clean EVERY electrical connection & consider upgrading to a dual field mid/late 70's squareback alt. adding an auxilliary 8 gauge lead from the alt batt stud to the starter relay batt stud is a good idea(w a #12 fusible link inline) & Nacho's plan of just running 2 leads in/out from the ammeter through the firewall bypassing the bulkhead is a good idea & as said Mad's info is excellent.

Rig up a light bulb socket and bulb with leads long enough to use as a tester. Use the device to follow the problem back towards the battery or from the battery towards the problem. If the problem shows at the battery you need a battery or have a charging problem.

Throw a volt meter on the alternator. Put the red lead on the big post of the alternator and black lead on the case of the alternator. Record the voltage. Now go to the battery and do the same thing. If you have more than a .50 volt difference you may have some resistance issues. Check your ground cable for high resistance. Check the connection and verify it is clean. Yoy should see at least 13.6volt for a good charging alt. I dont know how to full field this application but if you can find out the alt will put out its max. voltage during this test. There are specs im sure you can compare it with. You might just have a bad regulator also. They are only 20.00. Hope this helps. Tod

Thanks everyone.

I have the following:
Engine running - 13.6 at battery, 14.3 at the alternator.
Engine Off - 12.6 volts from the battery.