I think I should give a bit of history here.

Hi, I think I should give a bit of history here:
the 'rear fog light' dashboard light kept coming on whenever I applied the brake (not the handbrake). The problem seemed to worsen when I realised that whilst driving at higher speeds (e.G.70mph on the motorway) when indicating, the same light also flicked on and off in time with the indicator. ALSO there was loss of power in time with the indicator. I took the car to a mechanic, they told me it was an earthing issue brought about by a loose wire, they fixed it (said it took a LOOONG time to find, replace lots of cabling etc etc) and said I wouldn't suffer this problem again.

And now for the problem:
It has happened again, or almost exactly the same problem. When I apply the brake the light comes on. However now I seem to have limited power when driving at speed and it is only when I indicate that I get full power, once again this is in time with the indicator light blinking on and off.
I took it back to the mechanic, expecting it to be the same problem but they told me although it was an earthing problem it was due to the rear-passenger-side lighting not being waterproof anymore and the whole unit would need replacing. I decided not to let them do the repairs as I thought I'd go for a second opinion - That's when I thought of asking you guys. I felt a little suspicious when two such similar electrical problems struck so close to each other with such similar symptoms.
Does anyone have an idea about what I should do and if I could do it myself? If it was simply a case of replacing a part I would like to do it myself if pretty straightforward - I consider myself quite handy - but NOT A MECHANIC! I also would rather not spend even more cash on repairs. Would anyone recommend second hand parts if self-repair were possible?

Thanls for your help,
P