As Klaus suggests, the severed/shorted tailgate wiring can cause a myriad of problems.

That being said.... When you flip on the signal and it doesn't work, can you apply more or less pressure on the stalk and get it to start blinking. Or will it work in either the momentary or the constant position but not the other position? If the answer is yes to any part of this, you can bet there's a dirty contact inside the switch. It's doubtful, however, that you need to purchase a new switch.

Remove 3 screws from the plastic cover under the steering column. Separate the upper and lower covers with a putty knife or sharp screwdriver, being careful to not leave gouge marks in the gap, if you can help it. You don't need to completely remove the plastic covers. Just clam shell them open enough to reach the 2 screws in the next step.

Remove 2 tiny screws that hold the switch in place.

Pull the stalk straight out from the steering column with just a bit of authority and it will unplug from column.

Remove 1 tiny screw that holds a sort of greenish circuit board looking piece in the switch and remove that piece.

You will see printed contacts on the inside of the green piece that you can clean with alcohol and Q-tip.

If you can clean the little "followers", do that, but make sure when you are done that they still apply adequate pressure to the printed part so that there is still good contact. (If you're careful you can tweak the followers a bit to assure contact.)

Reassemble the switch, plug it back into to column, and test it before closing the plastic covers back together.

If all is well, make sure the the 2 screws holding the switch to the column are snug. Close the upper and lower plastic covers together, making sure they mesh properly and that the plastic grommets around the LH and RH stalks are in place properly. The upper and lower halves will sort of snap together. (I found this to be the toughest part of the job.)

Run the 3 screws up into the lower cover and you're done. You will be able to do this job faster than it took me to type it.

FYI - I've done this on both of our cars with good success. One of them was only about two weeks ago. (That's the only reason I remember the little part on the switch is green....... I think it's green.....
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)