Ascertain the motor itself is good first, by direct supply.
Classic common mistake of relying on what you're seeing!!
Due to the digi meter not really needing any(amount) of current(amps) to operate it can show full voltage, but the test light needs a little bit of current, not a lot but a bit, and more than a dodgy connection can give in this case.
It is likely just the switches, CONTACT points needs fixing, as the switch current, quite a bit in the case of windows every time they switch just before and just after the contact is made there is an arc of electricity that overtime dulls the contact, the reason they are silver plated normally.
Best thing to do is get some old knackered wet and dry of at least 1000 grit, so nice ans smooth and gently clean the contact points in the switch.
After this apply de-oxit, this will stop any further issues/problems, its awsome stuff, applied to the contact faces on the door switches it makes them work all the time for ever, as it stops corrosion, you only need one single drop, just leave on don't wipe off, it does the rest.
This kind of thing has had me scratching my head as well, that's why I now have a bald spot ha ha!!
tabetha