Diagnosing bad earth

[Rys]

I’ve got a bunch of electrical gremlins and I’m starting to suspect a bad earth. What’s the best way to go about diagnosing if that’s actually the case?

Current symptoms: fog light flickers when I brake even though I’ve cleaned up a badly corroded connector on the driver’s side rear light cluster; airbag + service light comes on despite having almost all airbags programmed out with a Clip (passenger dash bag still in, other bags all gone due to custom seats and wheel); boot solenoid has gone.

Any idea if all of those things can be related to a single bad earth, or am I just unlucky?

 

[Oggy997]

Jump lead from -ve terminal to engine block.

 

[JamesBryan]

Clean both rear light connectors.

And also check the earth which runs down from the passenger side rear light behind the plastic trim. Those black rubber boot stops next to both rear lights can leak in water and it runs down to that earth.