I've got a YM 1988 780 V6 (yeah, one of the two is OZ) and I've had fun 'n' games with the cruise control. But it follows the 740 way of doing things; the 88 760 is different.
The cruise control module itself in an 88+760 is located in the centre console relay bundle, to be precise. It's on the left-hand side bunch of relays.
(For 740s and 940s - and 780s for that matter - the cruise control module is located under the driver's side A pillar, tucked up and hidden from view. AFAIK 760s use a different module in a different location - the 960s certainly do, and 960s are closer to 760 than 760s are to 740s).
Here's a diagram (albeit LHD) of the 760 88+ setup.
https://charm.li/Volvo/1988/760%20V6-2849cc%202.8L%20SOHC%20B280F/Repair%20and%20Diagnosis/Cruise%20Control/Diagrams/Electrical%20Diagrams/
The parts catalog for the 960 lists both the factory-fitted cruise control parts diagram, and also a dealer-fitted / accessory type cruise control system. Some components are different. It may be the same for the 760, but I confess to laziness so you may need to check the 760 parts book yourself.
Hopefully, your car still has a working vacuum motor in the engine bay. They are available, but they're bloody expensive. Finding a good vacuum bellows at the engine end isn't easy; they were NLA new last time I tried to get one.
