I used one of these USB outlets that snaps right into the middle of an SRS plate out of an airbag car. I tied it to the cigarette lighter, and have switched 12 volts to charge phones without an adapter and it looks like it belongs there.
https://www.ebay.com/itm/12V-Car-Ch...ash=item2a96ffcbf7:g:vh8AAOSwl2haF49g&vxp=mtr
Pioneer radio with Bluetooth, that allows hands free calling and I can stream media from the phone to the car radio.
Replaced the vinyl seat covers with cloth seat covers, which took a while to find good ones.
I have a wagon, and I found a roll up cover for the cargo area and a plastic mat for the rear so I can keep the car clean when I carry dirty stuff from the junkyard.
Installed a suede covered steering wheel off of ebay. I am certain it's not a genuine Momo, and I don't care. It feels nice and it's a little smaller than the stock wheel.
I have a cupholder arm rest I bought from another Volvo guy. It didn't come with the bracket, so I welded up a little cargo pocket that I keep smaller stuff in. USB stick, lip balm, lighter, small flashlight, etc.
Put in power heated mirrors off of a 1993 car, but I don't think it was worth the effort.
Replaced all of the interior and exterior bulbs with LED bulbs.
Had the windows tinted.
E-code headlights.
A trailer hitch and wiring.
Plastic factory floor mats, or floor trays as they are called here.
I installed a constant hot 12 volt outlet in the bottom cubby where I plug in my gps so it stays on. I make a LOT of stops during the day for my work, and having the GPS stay on so I can plug in the next address without waiting for it to start is less frustrating.
I dig in the newer European cars in junkyards and grab stuff like spare tire bags, tool kits, etc. So I have a stash of tools and a pretty well stocked first aid kit in the back tucked into the spare tire.
A removable roof rack that clamps onto the roof gutters. This thing has carried longer stuff many times when I didn't want to lug the trailer around.
I pulled a cruise control system out of another 1991 car.
Lighted power window switches out of a newer car. I think it was a 740. It's been a while.
Dave Barton gauge faces, for the pretty.
I kept digging in the junkyards until I found nice parts to replace all of the loose or broken interior trim. This cut down on rattles tremendously. My exterior trim is still nice.