I made up a system of relays to tie the trailer pigtail into the car so the extra current flow would not exceed the limitations of the car wiring, or trigger the bulb failure sensor. The car's tail/brake/turn wires are available at the left rear corner of the trunk. Others familiar with your model year can advise as to which wire colours mean what.

For the cost and effort I probably should have gone to a hitch shop and bought an adapter. You will need some sort of adapter to connect the Volvo setup with its separate brake/turn lamps to the trailer, which nearly always uses the brake bulbs as turn signals.

General info here: http://www.accessconnect.com/trailer_wiring_diagram.htm

And a hitch shop can advise you on the right receiver choice.
--
Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.