It would be safer to place a fuse in series with the positive wire of any wires temporarily connected directly to the battery. Without a fuse, an accidental short at the other end of the wires could melt the wires and cause a fire. A 20 amp blade type fuse connected to the positive battery terminal could be used.

I have had several instances of non-functional windows on 1998 V70s. All the problems were fixed by disassembling the associated switches and cleaning the internal contacts.

The switches are a bit tricky to disassemble. Jewelers screwdrivers can be used to pry open the latches and lever the switch sections apart. There are some small springs and balls that you have to be careful not to lose. Switch contacts can be cleaned with a combination of fine sandpaper or emery board and scraping with a screwdriver tip. All the switches I opened had areas where the contacts arced and oxidized, causing intermittent or no connection.

Swapping rear door switches will tell you if the problem is only in the rear door switch. In some cases both the drivers master switch and the rear door switch have problems.

If you want to try cleaning the switch contacts, operate on a spare (used) switch first. That way you wont make the problem in your car any worse if a spare switch gets damaged.

There might be some YouTube videos that illustrate the switch cleaning procedure.

Mike