New owner

Hello all. I am new owner of a 1985. Please excuse my lack of knowledge if these questions seem trite. 
1. I have only driven 2 928s in my life. A 1993 and this 1985. both have had a groaning sound coming from the back (left rear as far as I can tell) especially noticeable at low speeds. Any ideas on what this is?
2. The door lights work but not the overhead lights. The radio light comes on but no sound. Are these related and is there a link on how to address it?
3.  is this site legit?   porsche-workshop.co.uk    I paid for a manual and the download wouldn't open and they never resonded. 
Thanks you!
Greek

Welcome to the 928 club

Does the groaning sound change as you hard corner the car?  You could be looking at a bad bearing.  Had the same issue on mine.  Not a bid deal to change and I don't remember them being that expensive.

Silly question about the over head lights, have you checked the condition of the bulbs, hopefully they are known good.  With a lot of 928 electrical issues, check all your grounds.  Don't think this would be related to the radio issues.

My experience with my 82 has been fuse connections on multiple issues. The system seems horrible and maybe just twisting a fuse will give good contact again.

Thank you guys for taking the time to help a new owner. The bulbs are good. Attached is a picture under dash. Seems I saw where someone cleaned the dial wheel to fix their problem? Does the right dial wheel control the overhead lights? any idea of what the twist knob is?

the groaning sound is not associated with turning. More noticeable accelerating at low speed.

The one wheel does the instrument lights, the other is for the intermittent windshield wiper, changes how often the blades move..  No clue what the other knob is, perhaps something after market.