Transmission overdrive switch fuse keeps blowing??

The most common problem that these cars have is the wire to the solenoid. The insulation will come off of the wire and it will touch the rear of the transmission. It is a bit of a pain to get to the wire to repair it if you don't have a lift.

Source: My 93 Vovlo wagon overdrive

More likely that the solenoid in the transmission is shorted to ground. Has any new electronic equipment been installed recently, like stereo, alarm, etc?

Source: 1992 volvo 240 keeps blowing overdrive fuses

YES! The owners manual states explicitly that "O/D Off" should be engaged whenever you are towing, I always ignored it for towing under 3000 lbs. The only way I would try it with our 4R100's is with a 4-5 plate Torque Converter Clutch and a Wood/IPT/BCS/Banks rebuilt unit running a separate cooler. The problem originated when Ford shifted to a Pulse Width Modulated (not just on/off) Torque Converter Clutch solenoid for "smoother" shifting. The increased slip in these clutches resulted in faster wear and higher heat levels... and blown solenoid packs. Another $150 down.
PS. You will not see any change in the tranny temp gage in the dash until the temp is well over 230F... too hot for Mercon! Get an aftermarket gage pod to keep an eye on things.

Source: towing with automatic in overdrive