First things first. Has the fuse panel been removed and cleaned up? If not, you have some uber-basic work to do. Remove, clean with dremel wire brush, install new VW/Porsche fuses. Do not use common Bus fuses, they don't fit. Post closeups of the panel before and after removal, you can tell a lot from the panel and the wire condition at each plug. Go to wiring diagrams (you have em, right?) or 928 specialists website for the 81 diagrams. Gotta do the panel refresh and clean the grounds before anything else to make the rest of your electrical troubleshooting worthwhile.
The headlight relay is pretty expensive; has a series of relays within. But its likely the cause as noted.
You need to understand and make the blower fan work reliably. Fuse 17, a blue 25 amp, protects the fan.
Relay XXI manages fan power and allows you to control with variable speed switch on the HVAC unit.
(next to last relay, top row, right side of your CEB panel)
Relay XXII manages fan power for defroster only (far right on your CEB panel board, top row)
Basically, the fan has two separate circuits that can operate it, which share the fuse.
If your fuse 17 is good and cleanly mounted, then you can jumper either of those relays for a second to determine if the fan itself works. Usually they do, but it might squeal or the whole plenum where it mounts might be full of mouse nests and debris. That's why, on reinvigorating a car like yours, I dive right into the fan remove / refresh / reseal before even trying the fan. Anyhow, if you are impatient, go ahead and see if it works with a jumper.
Further, replace both relays with new 53 versions from roger, $5 each.
The defrost should work automatically when the bottom slider is moved all the way right.
With both switches moved to the right a little bit, defroster circuit is off, but fan runs automatically on lowest setting with ignition on... even when selector knob is off. And you can choose different speeds with sliders moved. Its important that your fan is in good shape, lubricated and not blocked, otherwise it will drag and burn the fuse. That's why you should mechanically inspect and repair the fan, and lube it, and test it outside the car, and reseal so water doesn't leak into panel, first. Your choice how to roll. If you don't fix it mechanically first, at least snake a vac hose into plenum and a light and ensure any mouse houses are gone. The reason this is important is the resistor. Speed is controlled on the fan in this circuit by diverting power to a big old resistor in the plenum, accessible from engine compartment. Think "wires hot like a toaster" coils, which is what exactly it is. IIRC, slow speed means more coils glowing red, and exposed to mouse nests, on that resistor. I missed one on daughters car and it filled cockpit with smoke.
If your fan, once working, starts cycling off/on --- that resistor has a problem and has to be fixed or replaced. Just tuck that away for later.
Advice on looking inside center console is crucial. Notorious for mouse houses and chewed shorted wires. And bad stereo installs.
Its possible to troubleshoot the window a variety of ways, might just be a switch. Early switches can be easily dis/re assembled after cleanup.
Again, good advice given, can jumper the window at the 18 pin door plug, but lets save that for now.
Here's a thread that should help you with electrical / grounds work
http://forums.pelicanparts.com/porsche-928-technical-forum/586148-ground-cleaning-16v-euro.html Post #13 shows a panel...hopefully you are reading other detailed accounts of this type of stuff on rennlist or here.
Really like the fact you are saving this car. Good work.