For those of you concerned with keeping your fender hole intact (due to rust, fear of committing sacrilege against the Volvo gods or whatever), the back sides of these lights are plastic and may also be modified to fit the existing hole by carefully trimming some of the plastic material on the back with a razor or small file.

The goal is to be able to install the light with a snug fit, but not too tight as you risk breaking something on the light by forcing it in.

MODELS WITHOUT EXISTING HOLE:  The most difficult part of this install is getting over the fact that you are cutting into your Volvo fender!  The rest is easy.

On my '84 245 I started with a simple 3/4 inch (7/8 inch would be better) hole saw.  Drilling the sheet metal on a perfectly good Volvo fender with a standard hole saw is not a natural act and can be unnerving, but if you do it slowly and gently, you will easily cut through without any fuss.  A small sheet metal nibber is a good tool to have to elongate the hole horizontally.  If this tool is unavailable, a small metal file as mentioned above will work, but will take a little more time.  Your goal is to make a rectangle about the size of the hole in the red 740 above.  The hole will be about 3/4 to 7/8 inch high and about 1 3/8 inch wide.  The hole should be a snug fit for the light, but not too tight or you risk breaking some of the plastic on the back of the light if you force it in.  So careful, periodic test fitting as-you-go is necessary.  The side marker light simply snaps into place once the hole is the proper size.

WIRING: Most Volvo's do not already have a wiring harness in the fender for turn signal repeaters.  Some 850's do (possibly other later models too).  For you 240 and 740/760 & 900 owners, you will have to wire it yourself.

On my 240 and 740, I removed the wheel first.  I then removed the plastic inner fender liner, a very simple job with only a few screws to remove.  Then I simply ran two wires forward to the front turn signal.

You might need to drill a small hole in the inner metal fender if you can't find a place to pass the wire through to the engine bay.  If you drill a hole in metal it's a good idea to install a rubber grommet to prevent the wires from chaffing against the metal.

It does not matter which wire on the side marker goes to the 12v hot wire for your turn signal.  On most Volvos I've seen, one side had a green wire and the other side a blue wire.  The other side marker wire goes to the turn signal's black ground wire or simply connect it to a good chassis ground.  Simple plastic crimp-on wire splicers work fine for splicing into your existing wires.  I use an inexpensive 12 volt circuit tester to verify the wires I use.  This eliminates guesswork.  The type that uses a sharp point to probe a wire is best for this job.

WIRING:
There is more than one way to wire side marker lights so they will operate as signal flashers AND/OR running lights.  

Daniel Stern Page Marker Flash
http://www.danielsternlighting.com/tech/markerflash/markerflash.html
 

BULB-OUT CIRCUIT CONCERNS:
I have done four cars to date (one 240, two 740's, and one 760) and no problems with the vehicles' "bulb-out circuits" have surfaced.  The key with this circuit is to maintain even current draw between the left and right sides of the car.  Installing these light as outlined here should not interfere with your "bulb-out" feature.

Here is Michael Swanson's 1986 245. These are side markers from a 1999 S70. One light had a broken tab so it needed a little adhesive but the other one that was intact fit perfectly and snaps right into place.