I finally hard wired one of my 240s Tail lights. Some lessons I learned that might save you trouble;
Its tedious but not hard.
Buy some electrical solder flux! I wish I had done this 40 years ago. It would have encouraged me to solder more and crimp less. I only recently learned there WAS electrical solder flux.
Volvo brand bulb holder metal is easy to solder IF YOU USE FLUX.
ALVO brand holders are made from some alien "metal" that is immune to solder and flux. I couldn't do it.
Use longer wire than you think you need the first time you do it.
Instead of using the double element bulb for brake lights which sometimes melts the taillight plastic,(while in traffic jams when you are in gear and have your foot on the brakes for long periods.) I used only one of the elements for a running light. I used a different existing lamp for brakes. 240s have plenty of taillights.
It is nice not having to worry about your tail lights any more, or to hear your friends tell you a tail light is out.
Also, I was not paying attention and wired my backup lights to the fog light switch! Hmmm..... thinking about leaving it that way.
Hope this helps someone.