Voltage limiter is acting weird.

1970 Challenger, no oil pressure reading, gas gauge goes up and then down and then nothing. Next day fuel gauge works then quits.
Is it the limiter or wiring issue?

well, hard to say. It could be wiring to the limiter or it could be the limiter itself. Gonna have to get a meter on it,or just swap in a new limiter and see.

May also be a flaky ground to the cluster,

Voltage limiter reduces voltage to the gauges down to 5V.
12V will fry the gauges.
Never had one go intermittent, last time one failed on me was many years ago.
Only the one problem out of 20-some old Mopars I've had over 40 years.
I'd suspect a poor ground or some loose connections from your description.

Where would I find the ground to the limiter?

A bad ground would likely be the dash ground, the limiter grounds through the dash ground. Could make the dash lights (and turn indicators) do strange things as well.
The 1st think I think about with the nearly any old Mopar electrical issue is a ground issue. Fixing grounds solves a lot of their electrical problems. Gene

The limiter supplies power to all 3 gauges. If ONLY the fuel gauge spiked, points to fuel sender unit as suspect. If ONLY the limiter wer acting up; you see weird fluctuation on ALL 3 gauges.

Oil pressure was going up and down and now shows no activity.
Fuel goes normal and now keeps fluctuating.
Sometimes on and sometimes nothing.

Replace the voltage limiter. I strongly suggest getting an electronic replacement and be done with it.
RTE sells electronic limiters on Ebay.
Also MR.Heaterbox builds and sells electronic limiters..

Just to be absolute sure...with a test light and the key on ACC or RUN,pull the wire from the sending unit,ground your test light and poke the plug for the sending unit. It should be dim and pulse on/off. If that does not happen the limitier is fried.
You can test the wire for the oil pressure and temperature gauges same way but with the key on run only.
Hope this helps,good luck.

Thanks this is what I'm looking for.

I was going over an old invoice when I bought the car and had it at a very reputable electrical shop and they did fix all the grounds and determined there was an issue.
They did fix it but now the problem reappeared.

This, tho if one gauge works & the other doesn't it would point to as mentioned an intermittent ground or an intermittent open but for sure get an electronic version rather than the OE electromechanical type which will wreak havoc if it ever full fields.