The voltage when nothing is working is about 7V and when I turn on the motor the voltage goes to 6.5-6V. I think that this is preaty normal.
No that's not normal.
If a 11v supply is giving 7v when nothing is connected to it it is broken. So all bets are off with regard to current capability.
Make sure you haven't mixed up the AC output with the DC output and blown something.
No it is wired correctly! I find that voltage strange too but it seems to be working normaly - at least the motors are running without any problems.
This power supply has been modified by someone a little because it had some problems (that is why it wasn't put in the caravan ;D). Some of the components were removed - from what I was told a voltage regulator and some other things - I don't know what. But those modifications were made by an "experienced person :D". But it realy doesn't matter - it can drive three motors at the same time so current capacity shouldn't be a problem? The strange problems are relays that are not working correctly (which seems preaty normal because they are 12V not 7) and are breaking down.
For the past few years this has been powering some pumps and lights in a fish tank 
- without any problems. So I think that it is safe to use.
So any ideas what to do?
If not I will just buy a few 9V relays and hope it works. It isn't the best solution but I don't have much choice.
The strange problems are relays that are not working correctly (which seems preaty normal because they are 12V not 7) and are breaking down.
Sorry I don't understand that. A 12v relay will probably not work on a 7V supply, you can't guarantee that a 9V relay will work off a 7V supply either.
I am not sure what you mean by "breaking down" this is normally a phrase to indicate that the contacts on the relay are conducting when the should not be. Do you mean just not working?
Relays work for a week/month... But then after some time they stop working. And in a month three relays stoped working. And because they are used few times in a day that doesn't sound normal.
What I find strange is the way they work. So normal relay would do that:
click..............click
but these relays do that:
click..click...click...click
And I don't know why. This could be because the power supply is only 7V. But I don't know.
but these relays do that:
click..click...click...click
That would suggest to me that the relay is pulling in but the power supply can't supply the current and so the voltage drops. Then the relay drops out and takes the load off the power supply. The as the power supply has no load the voltage rises again and the relay pulls in and so on. click..click...click...click
Relays work for a week/month... But then after some time they stop working
When a relay has stopped working can you still measure resistance through the coil? Or have the contacts fused up so they don't open and close. This is not normal behavioured for a relay and suggests they are not rated at the voltage you think they are.
Yes I can still get the resistance. It is about 82 Ohms. And the contact is behaving as indicated on the sticker. So when there is no power on the coil one of the pins conducts electricity and the other doesn't. And if I put voltage on it is still cliks.
But you said it stopped working, I am missing something here what are you using to drive the relay, transistor or FTE?
Yes it doesn't work!
So I find this strange too. It is clicking, and behaves practicaly like normal. But it doesn't work. So I replace the relay and that one is working. So someting is wrong with the relay - even if it seems to work.
As for driving the relay: I am using a transistor!