My 1997/98 wiring diagram shows the tow hitch wiring connector is a 10-pin connector located by the power antenna motor assembly. You will have to remove the felt panel on the left side of the trunk (or whatever covers it up in your wagon).

And you will have to see if a Volvo dealer has the matching connector to mate with it. This would probably have the pigtails to connect to your particular hitch plug. It should also have the identification of which pin and wire colour does what.

If you can't get the proper Volvo connector and want to jury-rig something using ScotchLok connectors, post again here and I'll reply with wire colours and pin ID's on the car connector.
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Bob: Son's XC70, my 83 240, 89 745 (V8) and XC60. Also '77 MGB and some old motorcycles

Uncle don57,

I sort of imagined you'd get response by now. I can't provide an exact response. Just some pointers ...

Volvo Cars AB may have sold a Volvo 940/960/S & V 90 wiring harness kit that may have been sold as part of the Volvo factory trailer hitch and receiver 900 series. Maybe variation by model.

Volvo autos sold with the factory or dealer ship installed trailer hitch and receivers spliced into the taillight writing harness using crimp connections to electrically secure in the trailer harness. On older models the harness merely remain spooled up for connection to the trailer wiring harness through a water proof connector. Usually the wires were secured under the truck lid.

On newer models, there may be a connector plug that secures someplace to the body near around rear bumper so the trailer wire harness connector jack secures to.

If wiring up a connector trailer that electrically connects to your S & V 90, be mindful of quality connection that preserves proper continuity and a balanced potential between the left and right side of the trail light wiring circuit. If not, your bulb out sensor will detected an unbalance between the right and left side of the lighting system, be it brake, turn signal, or running (parking or position) lamps.

You may want to research the usual vendors like Tasca Volvo, FCP Groton, and iPd and your local Volvo stealership for guidance. A Volvo factory wiring harness for trailering may be available from Volvo.

Else, you may be able to use a quality after market kit for trailer connection. One that offers enough circuit connections for the S & V 90. You'll also want to include a dedicated circuit for ground between the S/V 90 and ground.

Search the:
https://www.brickboard.com/FAQ/700-900/
http://www.volvowiringdiagrams.com/

Also, search this forum using the search feature or use google and target brickboard.com by adding "site:brickboard.com" (no quotes) at the end of the search string for brickboard.com articles that include words like trailer, hitch, and kit, perhaps.

Hope that helps.

Sat-Your-Day Morning Mac Duff.