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I've read what I wrote a few times, and I'm not sure I even understand, and I did it, but I can't think of a better way to put it. But here it is:
Just did this to my 81 a month or two ago. Despite the complicated explanation it is not hard at all if you have too much free time and wiring diagrams (Bentley works well).
The headlight switch is different -85 and 86-, so you will need the newer switch. You will also want the plastic dash part the switch mounts to, I didn't get that. It is the black plastic part that the headlight switch, instrument cluster, center vent, and 52mm gauges bolt to (a picture would be better, but I don't have one). Not the trim, behind the trim that pops off. It isn't vital, but the switch has a little pin that aligns it in that dash, which the early dash does not have. If you don't get it then your switch will work just fine, but will be rotated about 30 deg clockwise, so when in the off position it will be pointing at the parking lights pictogram. Lastly you'll need some spade terminals and wire. 84's SHOULD have a headlight relay (in addition to the high/low beam relay on the fender). I added one to mine years ago, a dual terminal 40amp unit from an Audi I think.
The early switch is pretty simple, constant +12V in and then out to parking lights or parking lights & headlight relay (or the High/Low relay on early cars). The newer switch uses unswitched +12V to power the parking lights only when the switch is in the parking light position. It uses switched +12V to power the headlight relay when in the headlight position (the switch itself does not power the parking lights, the relay powers both the parking lights and the headlights).
You need to add a wire from a switched +12V source to the switch, Volvo uses the power side of one of the fuses (wiring diagram will show you which one). That is the most painless option I could find. Then you add a wire from the spare output (87 or 87b) of the headlight relay to the power side of the parking light fuses. Reuse the wiring from your old switch for the unswitched +12V in, and the outputs to parking lights and headlights.
When I say power side of the fuses, I mean the terminal way behind the rest, some of them are empty and some of them have wires with a plastic shield over them. You might have to unscrew the fusebox in order to get all the way to them. I had some of those plastic protectors on hand and used them. Safer that way.
Your headlight relay must not conduct between the two 87 terminals. If it does then the headlights come on when the parking lights do, and when the headlights do. Not ideal. Took me a while to figure that out. Test the relay on the car, not one you have in the house that looks identical!
When you're done the headlights will only turn on in Run and Accessory key positions. The parking lights alone will turn on regardless, but once you turn to headlights they switch off too. Guess it could be useful if you're stranded or waiting for someone.
If I were to do it again I think that I would simply replace the unswitched +12V power for the older switch with a switched source. Move one wire, avoid a lot of headaches, and when the key is off the parking lights are off too, so you really can't run the battery down.
Good luck.
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Logan
1981 242GLTi 1967.5 122S Automatic (project)
Seattle, WA
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