Where the mustard relay is, opening the mustard relay and resoldering etc. is described on Johan's page
http://bmwe32.masscom.net/sean750/mustardRelay/mustardRelay.htm - http://bmwe32.masscom.net/sean750/mustardRelay/mustardRelay. htm
and E32fixes.com
http://www.e32fixes.com/results.asp?method=show_fix&fixid=1 - http://www.e32fixes.com/results.asp?method=show_fix&fixi d=1
So I will not go into detail for this.
Repaired 2 mustard relays yesterday.
While one relay was o.k. after resoldering, the other one still had a fault message "tail light", although the tail light was o.k. So tested the relays on the soldering contacts with a DMM (Ohm) and found faults.
De-soldered the 2 relays and used a small strip of very fine emery paper to clean the contacts of the relays. Very often a bad contact there is the cause of the fault. But still the fault message came up in the check control after testing it on the car.
Finally it turned out that the cause was the pressure of the contact on the relay, it was not enough or maybe damaged to make poor contact.
Bent the lead of the contact a little bit to give more pressure when relay was activated.
Checked it again on the car and it was o.k.
No more alarm.
As far as I know only E32 with LKM-L have a mustard relay, while E32 with LKM-B have in the same position a socalled daylight relay. That is much less complicated as it just has the blades and the connections, no electronic inside.
Here you see a mustard relay and the daylight relay
http://s71.photobucket.com/albums/i155/shogun_bucket/?action=view&current=Senfrelais.jpg&refPage=&imgAnch=imgAnch1 - http://s71.photobucket.com/albums/i155/shogun_bucket/?action =view&current=Senfrelais.jpg&refPage=&imgAnch=im gAnch1
You can see the connections and the missing ground pin on the 'dummy mustard relay"

So not always soldering will cure an mustard relay.