Date registered: Oct 2013
Vehicle: A200 Sports Coupé C169 2006
Location: Bozeat, UK
Mentioned: 2 Post(s)
Quoted: 207 Post(s)
The fact that it's working between the two positives, tells me there's a short somewhere, and also possibly a bad earth.
I would trace all three wires and ring them through, starting with the ground one.
Also check the fuse 38. A cheap voltage indicator, plugged into the power socket might be useful too.
The ground point for the front power socket (for the LHD car) is W37 - it is about 15-20cm off the floor on the front of the driver's door frame - see attached file.
Sometimes, when the water drains get blocked, water gets inside the cabin and rises to the ground point, which in turn gets badly corroded.
Check your drain points, and check your main ground points (I'll get the other files on W245 ground points in a moment).
As to the brightness of the T5 bulb - they really are not that bright, so perhaps an LED replacement is a good option.
Also, see if it is brighter, when tested on a bench - if it is, this would be a good indication that, there's a bad wiring somewhere.