Hi,

when unlocking my car in the dark, I sometimes get both mirrors to turn on their approach lights... and sometimes they don't light up.
Most of the time I double-lock my car to get the alarm engaged.

What is the rule to get these lights on or not ?

 

This last bit leads me to suggest this: Check your key FOB battery and/or try a different or new remote altogether.

Our XJ behaved very erratically when locking/unlocking until I realized the key FOB was simply worn out. I would have to push the lock button a minimum of two times, usually more. A new battery didn't do much, but a new remote solved everything.

Available faily inexpensively on ebay.

 

This happens on both fobs, batteries are fresh.
The car unlocks allways the same way ( doors unlock, mirrors move to the driving position).. but the lights sometimes light, sometimes not... it allways the same darkness...
So what is the criteria for the mirror lights to be switched on ?

 

They won't come on if the ambient light detector says it's daytime. Of course it could be corrosion in the mirror housings. Have you had a look-see ? I had one work and one not. I found there was corrosion in the bulb holder, cleaned it out and it has worked ever since

 

Is this the same detector, which controls the xenon-headlights ?
They work perfectly by day/night...

Or is it a different sensor ?