Looking for a common chassis earth

I think i’m looking for a shared chassis earth.
Basically my blower motor isn’t working and the radio trips out and asks for a code.
I can get the radio working again with code 0000 but if you ask it to eject a cd or play with any functions it trips out also the speakers intermittently are inop.
The blower motor works fine when bench tested…connected direct to a slave battery…i also had it working correctly i.e. changing speeds last week when connected to the car with a patch lead,
I’ve had to leave the car for the last four days due to having new windows fitted so i’ve only been able to devote and hour of my time to it this evening.
I can’t get a solid earth from the blower motor multiplug using a test probe as it very briefly shows an earth then disappears.
I suspect a poor or corroded earth to chassis, but where?
Any help appreciated.

Edit, 2005 LE NC.

You need to specify which model you have.

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Apologies, this is my other MX5 which is a 2005 LE NC.

Don’t know if this is any help, maybe one of our electrical gurus can translate it

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