First of all, thank you for the replies!
@ Greg G: I changed a year or 2 ago the lights with new bulbs... difference was noticable, but it was still very dim... I'm quite sure my original electrical system works fine and is grounded decent.
That is why I'm thinking to switch over to LED's...
Nonetheless, highspeedmotors was indeed the shop I contacted without receiving answers... too bad, because I found out it was the only supplier for 6V positive ground LED's...
@ Dennis_MN: I looked at the link you've sended, but they don't specify if it is positive or negative ground...
@ MoparAl: please let me know indeed... (they were 6V positive ground??) UK is closer to Belgium as the US
Cheers 'n greets