Not sure which Tremec you have, or which Volvo...

On my T-5Z (by Tremec), the reverse light switch is on the passenger side of the tranny near where the speedo cable hooks up. It screws into a threaded hole in the side of the transmission. It has two terminals on it. When the transmission is in reverse, the switch closes, and current can flow from one terminal to the other. I ran a 12V+ wire from the fused connection that was marked for the back up lights to one side of the terminal. I located the wire for the back up lights by checking the color in the trunk and tracing that up to the fuse box (confirming the color with my repair manual) and ran it down to the other terminal on the reverse light switch. I ran both wires through the firewall, and down the tunnel to the tranny. Works like a charm.