My 1990 1.6T dies when put in any gear.
I tried to get rid of unnecessary cables including diagnostic box, found out there are grounds there for safety / neutral switch causing my car to shut off immediately when put in gear. Can anyone help me with this problem?

 

There is a wiring diagram on the post you necro'd from 12 years ago with a wiring diagram that may help you. Otherwise spamming your question in every section is a good way to make sure nobody helps. Generally you're not going to get help on dumb stuff you do to yourself and then not be willing to put in the work to fix it.

General rule to live by: If you aren't prepared and have the knowledge to 100% rewire your harness you probably shouldn't be clipping "unnecessary" wires.