Problem with passenger side window motor

I have had a problem with the passenger side window motor not working on my 2004 MG TF. I did the usual checking of the fuse and then checked the connector to the motor and there is voltage coming through so all good. However the motor still didn't work so I bought a used one off eBay for £25. This motor works as I tried it out before fitting. On fitting it won't work. Checking the voltage again I get voltage before connecting and when I connect one wire, but when I connect both no voltage comes past the connector. This has me confused. Any help would be great before I have to get an auto electrician

How you measure the voltage? My assumption is that when you plug in the connector it is either not pushed enough or pins have extensive play and not making connection good.
You should measure voltage on the same place before and after connection.

Using a multimeter. I measured by taking the plastic covers of the connector to get a good connection and saw there was voltage before fitting the motor, but when I connected the motor there was no voltage passing through the connector. The connection is a good fit. Very puzzling!

Could be BCU relay .

What kind of voltage you measured? You should have full 12V. If not, if you got lower voltage on the connector, then you do not have sufficient voltage on the connector in the first place.
Also measure the voltage on the window relay. Relay might be the problem.
Passenger window is controlled by BCU only by energizing relay coil.

My guess is a poor relay in the BCU. If you're getting 12v off load & the voltage collapses under load (i.e. when the motor is run) this would indicate the current supply capacity via the relay is compromised: grotty relay contact.

Relay within BCU controls only drivers side window, not the passenger side. For the passenger side window relay is outside BCU, within fusebox under steering wheel.

I didn't know that, thanks, Mike