adurer

@dohnjoe
You can make your own frequency divider by 2 very cheap and easy (see schematic below). CD4027 allows input signals of up to 15V. The output signal is TTL. You can use the 7805 IC to get 5V.

No problem.
The cable from alternator B+ to battery positive should have 10 mm2 in section. That corresponds roughly to minimum 3.5 mm in diameter. Remove those two cables and install a single tinned copper cable 3.5 - 4 mm in diameter. A 3 Volt drop is huge. Present cable(s) don't short to ground, you would have used a fire extinguisher by now. It's just corrosion inside.

This is how it should look like. I remember having to insert one more O-ring into that gasket to keep the cap from rattling.
 

When did you clean the throttle body last time?

According to ETKA, for FAVORIT and FORMAN the part number is  6U0611053B. You have 19.05 mm diameter wheel brake cylinders.
 
Favorit PICK UP, FORMAN PLUS, and PRAKTIK have 22.2 mm diameter wheel brake cylinders (part number  6U0611053A)

I have annotated your photo for better understanding (see below).
 
For cars up to 07.91, the fuse diagram is as follows:
 
1 15 A interior lights, brake lights, horn, hazard lights
2 8 A front and rear wipers/washers, reversing light
3 8 A directional lights, dashboard lights, carburetor shut-off valve
4 15 A engine cooling fan
5 15 A rear window defogger
6 15 A blower motor (heater)
7 8 A left front and rear side lights, license plate light
8 8 A right front and rear side lights
9 8 A left dip beam
10 8 A right dip beam
11 15 A front and rear fog lamps
12 8 A right high beam
13 8 A left high beam
14 15 A not used
 
You have a short between fuse 4 and fuse 3 circuit. I would take a closer look to it, cooling fan is now overcharging another fuse.