Does anyone know if that transistor can be tested while installed or if it has to be removed to test.

 

If your speed controls only work on high, the transistor and/or associated resistors/capacitor circuitry is almost certainly kaput. If you go to the trouble of removing the blowers to test the transistor etc, you might as well replace all the components because the cost of the bits is fairly minimal.

Here's a link on Jag-Lovers that I used when I repaired the blowers in my own car:

Jag-lovers' Ebooks

cheers

Larry