OK let's look at this logically
Q/
Should both brake lights be on at the back when the headlights are turned on???
A/
NO. when the headlights are turned on there should be no brakelights on, unless you have your foot on the brake.
The Headlights and Brakelights work independantly of each other.
When side lights or headlights are on then both tail lights should be on.
When braking both brakelights (plus any centre auxilary lights) should be on regardless of whether the head/side lights are on or off.
There is on the off side rear a suplementary "bright" light that is dependant on the dip headlights being on to illuminate. This is an "anti collision" or rear fog light and should only be turned on in cases of extreme poor visibility such as heavy spray or dense fog. When this light is on there should be a warning light visible somewhere in the cockpit, either on or near the light switch, or displayed on the speedo dioal or somewhere similar.
Q/
Both brake lights are fine when I touch the brakes.
A/
Well that's good as that is exactly what should happen!
Q/
. . . .only one is on permanent when the head lights are on.
A/
That does seem eirily like a rear fog "anti collision" light now, doesn't it? Could that be what it is, possibly? Just a thought?
Q/
A/
No, not really.
Q/
is this normal fellow GTV drivers???
A/
Yes, this is normal to all drivers.
Now go and find the switch and turn the thing off.
P