There is a reverse switch on the top of the transmission, under the airbox.

I've done this and there are instructions over at Volvospeed:

http://volvospeed.com/Repair/tranbackup.php

It's an easy Saturday morning project while listening to "Car Talk"! As I remember the most complicated part was snapping the airbox back in place.

-BTC

'98 V70 T5 5-speed, 175k mi, heated velour cloth comfy chairs, factory HD suspension, Bilstein HD, front IPD stabilizer bar, Volvo strut tower brace and skidplate, XC grill, C70 jewel E-Codes, V-1, Lidatek, IPD stuff, JB Weld, Original Duck tape, zip-ties, two Honda license plate nut impressions in back bumper (gift from daughter), Mobil-1 since new.

"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." - Douglas Noel Adams (1952 - 2001)