This is off of my experience with a w126, there are probably some differences between the years so these are only "ballpark" suggestions. Someone with more experience will chime in.

1. Tach could be related to fuses, check/replace them all and make sure there are no hairline cracks. It could also be the tach amp if they still used them in 1987. That's the little black plastic "garbage can" behind the driver side headlight.

2. Do the rest of your dash lights work, such as window switches, climate control, etc.? If no, then it could be the rheostat, which is known to go bad. You'd have to pull the cluster to get at it, but you'd have to do that anyway to get to the light bulbs if they are burned out. You can solder in a jumper wire to bypass the rheostat if you choose.

3/4 don't know

5. Check the switches, they are known to get gunked up over the years. If that doesn't work, take the door panel off and check for power at the window motor. If you have power and the switches function, take out the window motor and rebuild. All 4 of mine were seized up, all it took was a cleaning-out and re-lube and they all work well now.

6. Vac lines probably contributing to your cruise control issue, and possibly your A/C, depending which ones are missing. Pick up a factory shop manual if you don't have one, it will help you tremendously with vacuum. There are also vacuum diagrams available on this site (sticky at top of forum) that can help you troubleshoot your mess under the hood. Also, a mityvac tool will assist in tuning the vacuum system and tracking down leaks. Welcome to the forum!

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth