Technical heater turns off when i turn my lights on?

[bilboblades]

I have a problem with my 2003 multipla when i have my heater on it works gr8 but as soon as i turn on my lights the heater turns off even the the lights on them stop working, Any 1 had this problem and fixed it ? if you have how did you fix it ? iv'e been looking over the net and the only thing i have found is they say its a common fault on fiats and its a wire that needs earthing but which 1 and where?

 

[momoe]

Try this link https://www.fiatforum.com/multipla/...r-fan-not-working-please-help-if-you-can.html

 

[bilboblades]

Sorted i found that some 1 had cut the black wire and earthed it but did not conect the wire going to the heater