Serious electrical issues....help!!

[DuncanDonutz]

Oh man, I don't even know where to begin. So, a couple weeks ago my starter and/or solenoid (or so I thought) started going out. The engine would turn over real slow but still starts. Battery and alternator are brand new and working fine. I kind of ignored the problem as I didn't have the time or money to replace the starter yet.
Yesterday, the fuel and temp gauges stopped working (warning lights and tach still work) and my radio would start turning on and off at random, and in sync with my blinkers. Also my headlights were dimmer than normal and did not change brightness with engine RPM. Now, as of today, if I turn my heater on or push on my brakes the idle bogs down. At first with the brakes I thought I had a vaccuum leak or a bad brake booster but when I noticed it with the heater too I now question that. I checked all the grounds in the engine compartment (chassis ground by battery, transmission and cylinder head) and cleaned them up, made no difference whatsoever. Then I started to take the dash apart to see if there was any obvious breaks in the wiring back there or anything. The wiring in this car is HACKED TO PIECES. A bunch of chopped up old wires behind the stereo (I assume this is the factory stereo wiring) Butt connectors everywhere, there's two relays that have been added, one of which is jumped with a wire. I have no idea what these are for as they aren't in any of the Bentley diagrams. Please please help me, I have no idea where to even begin looking for a problem. I'm not much of a wiring guy so any help is much much appreciated. I will post pics in a few mins when my phone is charged and I can snap a couple.

[antichristonwheels]

get a new harness from a junk yard

[cuppie]

To this, I'll add:
Replace the battery cables.
20+ years of corrosion / degradation of them will play hell with the car's electricals. And, you'll defenitely notice it in terms of starter & alternator performance (no amount of body harness issues will cause slow cranking - that's all battery, starter, batter cables, cable connections.)

[DuncanDonutz]

Thanks for the tips guys. Im going to be off of work all next week so I plan on completely taking the dash apart and hunting down any shorts or damaged wires. I'll probably decide then if Im going to completely replace the harness--none of the junkyards close to me have any Mk2 VWs with Digi 2 right now, they're all either Mk 1s or have CIS. I don't imagine these harnesses being swappable but maybe I'm wrong.

[AnAgentOrange]

When you replace that harness and battery cables, clean all those ground points. Effed up grouds will screw with you to no end...

[DuncanDonutz]

I was thinking about it while I was at work today, and...is it really better to go to a junkyard, spend all that time pulling a harness I'm not even sure is good, or just fix whatever short circuit there is behind my dash? Or is it possible to source a new or lightly used harness somewhere? Haven't had much luck googling it.

[ziddey]

Yeah some GTI/GLIs had Aktiv or whatever "high end" sound system. That's almost always going to be hacked up.

Have you tested your voltage while running yet? How long have you driven since the new battery and alternator?

[Fat Rabbit]

First thing I'd do is load test the battery. Then I'd pull the alternator and have an electrical shop test it. I've seen plenty of bad "rebuilt" alternators over the years. Sometimes if the diodes in the voltage regulator get messed up strange things can happen electrically. New Voltage regulators can be had on line for $20 or less and are easy to change (Just two screws.). Those two things, checking the battery and the alternator, are cheap and easy. Certainly a lot easier than messing with the wiring behind the dash. FR

[DuncanDonutz]

Thanks for pointing me in the right direction Fat Rabbit. Upon further research I'm thinking "alternator ripple" might have something to do with this, i.e. one or more diodes are bad and the battery and electrical system are receiving excess AC current that's making everything wonky. Had never heard of this before, thanks again! Will have the alternator tested at local auto parts store.

[DuncanDonutz]

Had the alternator tested, was only putting out 11.4 volts. Took it back to NAPA and got a warranty replacement. I hate remans! This is the 3rd alternator I've been through 3 in less than a year!