I have an '88 760T and the relay is on the drivers side center mounted on a flimsy metal bracket on the topside of the engine just in front of the ABS solenoid. There are two relays (black plastic boxes) about 2" x 2" maybe an inch thick... After I finished doing my 250K mile service I stopped at our local Volvo dealer and asked one of the lead mechs for any additional items that I should consider... When he asked me about these he about fell out when I told him they were the originals...! I replaced both of them ($40) per... He also told me that if one were to ever fail, as evidenced by the engine just shuuting down, you can use a headlight relay to get you going as they're interchangeable... I just kept one of the old ones in the trunk for a back-up...

Frank

here is a link to the 700/900 FAQ section that details how to get to the
fusebox to replace the relay...........

http://www.brickboard.com/FAQ/700-900/ElectricalCircuitsRelays.htm#FIRelayApplications

Fuel injection relay is located on the board with all of the fuses and other relays. It is behind the ashtray. Remove the ashtray and the lighter. Remove the small plastic cover around the hole where the cigar lighter goes. There is a screw that you can remove take out the tray between the ashtray and the radio.
You should now be able to access the relays easily. You may will still need to pull out this board to get to the FI relay. It should be a relay with a white plastic cover. Some refer to this as the fuel pump relay. I think it is the same thing.