Both CJs and FSJs have the CVR in one of the gauges, in the fuel gauge and temperature gauge resp. Nominally the CVR makes 5 volts to run the gauges at that reduced voltage. If that's the problem, I'd suggest searching for threads here about replacing the Jeep CVR with a 7805 or 7806 integrated circuit. I've done this to my J20; pretty easy and it works great.
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk