Thanks for the input, Baffler.
Well speedometer lights go dimmer/brighter occasionally, but that's maybe something else since one section is totally dark sometimes. I should check the lamps. And quite recently I noticed that the blower motor sometimes when turned on full power speeds up little after a while.
When you say "rad" you mean radiator, right? And the corrosion of the loom happens most propably just there under the radiator? (
Edit: After a little research the answer to both of these is apparently "yes".)
I'll try that fuel primer pump trick.
Does the red arrow point to the shut off solenoid? It has a rubber cap on it secured with a cable tie and if I remember correctly there was a little diesel under the cap. How is it sealed and anything special to consider, if I was to take it out?
I changed the alternator brushes. There wasn't much left and the slip rings were quite groovy, too. Didn't notice any difference on test drive, though.