Pre-glow plug icon turned on after swapping the transmission from a Jetta

Hello,

Is the glow plug icon flashing or staying on solid?
If it is flashing it could mean that the engine management system has set a fault not necessarily relating to glow plugs.

Any of the below faults could be stored, so it is important to scan the vehicle using suitable diagnostic equipment. For this vehicle VCDS is the best option.

00741 Brake pedal monitoring - implausible signal
01044 Control unit incorrectly coded
16705/P0321 Engine speed sensor - implausible signal
16706/P0322 Engine speed sensor - no signal
16955/P0571 Brake light switch (F) - implausible signal
17653/P1245 Needle lift sender (G80) - short to earth
17654/P1246 Needle lift sender (G80) - Implausible signal
17655/P1247 Needle lift sender (G80) - Open circuit/short to positive
17762/P1354 Modulating piston movement sender (G149) - electrical fault in current circuit
17969/P1561 Metering adjuster (N146) - control difference
17978/P1570 Engine control unit blocked
18020/P1612 Control unit incorrectly coded
18026/P1618 Glow plug relay (J52) - short to positive
18027/P1619 Glow plug relay (J52) - open circuit/short to earth
18040/P1632 Accelerator position sender - voltage supply

A best guess would be that the TCM on the replacement gearbox has not been coded to the vehicle, code 01044. But this would have to be verified first.

Do any of the scan tools you have tried have any communication with the vehicle?
If not then first check that there is 12V present at pin 16 of the diagnostic link, OBD connector.
This can be done by using either a multimeter or test light.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Please let us know how you get along.

Cheers, Boris