Follow Dan's advice first.

Then tell us...

Auto or standard transmission?
What year?
Station wagon or sedan?

Sedan rear light assemblies are more prone to problems than the wagons. A torn circuit "board" could create a short.

If yours is a sedan, I'd check the rear light systems.
Remove the cover panels. Eyeball the "circuit boards", check for tears or burn marks. They are flexible plastic sheets, I never want to call them boards.

If no obvious problems found, test them by eliminating them from the circuit. Disconnect both nylon connector clips at the boards. Now put car in reverse and see if fuse blows.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.