You're right, you can use a GP relay from a 300D as the engine is the same. I replaced the relay on my 240D after being unable to repair it or find a cheap used one. New ones are only $99 and they have afterglow, making any cold diesel run smoother, plus they come with 4 GP's as a bonus. I had the same problem as you. The relay would only energize for barely 10 seconds which isn't long enough.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL