Hope all of you are well,
I have an issue with my 124 spider. I have finally replaced the ECU and engine started to work fine, however I needed to install a windshield washer to pass the road licence test and after doing so and trying out the washer all worked fine. Unfortunately once I started up the car it ran for a number of seconds and died on me.

The washer motor was connected onto a loose 12v wire I found running around in the engine bay, that I traced back and saw it was connected directly onto the + side of the alternator that I assume is connected directly to the battery since it's a constant live wire .after this I installed a bypass switch in the cabin, as didn't want to tamper with the dashboard and I earthed the negative cable to the body.
I am very dissapointed that I may have caused a problem and obviously now again I am lost and have no idea what way forward is best to troubleshoot the issue.
Has anyone of you ever passed through an issue similar to this? Would you know of a way to check and what to check first as I am very concerned and would like to resolve over the weekend.
Thanks you in advance .and wish you all a great weekend.