[chittychittybangbang]

Is there a way to make it longer without a vag com cable?

not that I am aware of. If you want to do any serious work on your car or want to get access to diagnostics, etc., a ross tech vag com cable is needed.

PS, the mk4 cars for some reason, tend to have shorter glow plug duration than the mk3 cars. Don't know why. The one thing you can do is to turn the key off and back on to cycle the glow plugs again.

Another thing you may want to consider is the glow plugs may be aging or the harness could be bad. Glow plugs get very hot and do wear out slowly.