there is a reverse light sensor on the gearbox, when you stick the car in reverse this sensor allows continuity through it which illuminates the reverse light.
when you are in any gear there is power going to the reverse light, this could be because the sensor is stuck so power can pass though it at all times (would usually mean light stays on in neutral too) or you have power going direct to the reverse light wire and bypassing the sensor (should also mean light remains illuminated when in neutral).
the interesting thing is that the light dims when you take your hand off the gearstick, this suggests that the light is earthing through your hand, which is impossible because the gearknob is rubber so it will be insulated (have you fitted a metal gearknob?) and the light is not supposed to earth back to the gearbox. so i would then think that when you apply pressure to the gearbox it is sending current to the whole gearbox, making the gearbox casing live. that should blow a fuse, or kill the battery, unless the earth strap on the gearbox wasnt connected, which could also cause problems like this in the first place.
i think the problem is to do with the earth. i'd check that the earth strap between the gearbox and car chassis has been fitted, and it fitted securely, and the surfaces are not corroded. i think when you sort the earth our both the speedo and reverse light will work ok. fingers crossed
