The only thing that determines the charging voltage is the regulator in the alternator. IIRC there are three wires for the Delco alternators - one to energize the field at startup (also operates the indicator light, if present), one for the sense voltage that tells the regulator whether to charge or not, and the charging wire. I believe the sense wire makes a u-turn back to the charge wire, so there's no remote sensing. If the charge wire has a high resistance, this could lead to overcharging the battery, but that's unlikely. Just check that the connections are good, and that there is no measurable resistance between the alternator plug and the positive battery post.
If it still overcharges, then the regulator is suspect. You can buy a regulator and take the alternator apart to replace it, or you can buy a complete replacement alternator. Autozone and likely other stores have a test station where you can test your alternator.
Check what condition will give you a "full" reading on the gas gauge. If zero ohms is full, then you're likely shorting out your wiring somewhere. This could be connected with your head unit wiring. Some head units will maintain presets for a long time without battery power (using a capacitor dedicated to maintaining the memory), some need outside power. I suspect that the newest units use flash memory, and don't need any power source.
1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D