My name is Roger and I will do my best to help you with this difficult problem.
Can you send me the last seven digits of the VIN number for your Z3 as I would like to look over the wiring diagrams. Also are you familiar using a volt meter or a test light?
Thanks for the reply.
The last seven digits of my VIN are: LK17274.
I have no experience using a volt meter or a test light.
Thank you for the VIN as it helped me determine you have the M54 engine.
When you turn the key on do you get any instrument cluster lights at all? Also can you please check the battery connections again. Follow the grounnd cable on the battery to where it goes to the body and make sure both are tight and secure and the positive cable is secure as well.
Then I would try another vehicle and using jumper cables on the jump terminals under the hood see if the vehicle will power up with jump cables.
Let me know what you find. I am hoping this may be just a bad ground for the battery.
Looking forward to hearing back.
The plastic tube is a vent to allow the gases from the battery to vent out. The vent is in the body of the vehicle near the battery.
The ground is the negative cable on the battery. Check the connections at the battery and where the negative cable attaches to the body of the vehicle.