You need to remove the cover from the seat so that you can access and REMOVE the electrical harnesses from the switch(es). From the pic you posted...that is what's preventing the thigh extension switch from being removed.
All of the switches in the seat's cover are secured from the rear side of the cover...some are also pulled away from the rear side, and some like the thigh switch are released from the rear, then push out through the front. See pic below (from Rhett's heated seat DIY on e38.org) on how to remove the rocker switches (he's removing the upper articulating back switch...but the thigh extension switch is designed and released the same way:
Look at the switches (not the buttons) in Bluebees pics below...seen are the large seat switch (rectangle) and the round lumbar switch...both are attached to the rear. And if you look to the left of the large seat switch...you can see the 2 tabs that are holding the plastic cover where the ROCKER switch goes (#1 in diagram) that controls the thigh extension feature. The 2 way rocker switch don't have "removable buttons" so the whole thing is removed/inserted from the rear. The large regular seat switch is the only one with removable buttons that come off/go on from the front side of the cover. If your seats don't have one of the four features...then it has a "fill-in" plug...which is also secured from the rear. If you've pulled the switch out from the front without releasing the spring clip, you may have broken the cover and will need a new one for the switch to fit properly...so make sue the cover isn't broken.
The seat cover has a place for 4 switches
...which depends on what features your seats have...again ALL of them are inserted/removed from the rear of the seat cover.:
- 2-way (rocker) thigh extension switch (#1 in diagram but has a different part nbr vs the articulating upper back switch)
- 10-way basic seat switch (big rectangular one) (#7 in diagram for driver's side)
- 4-way round lumbar switch (#10 in diagram)
- 2-way (rocker) articulating upper back switch (#1 in diagram...but has a different part nbr vs the thigh switch)
When you see these diagrams like the one below...more than likely its from www.realoem.com
& if you go to that site to look up the diagram...you will see that there is a part nbr index below the diagram. So if you don't understand what you see in the diagram...look it up so that you can see what the part is...or if it applies to your car. If an item in the diagram is NOT listed in the part nbr index...then that part is NOT used on your car...especially if you used your car's VIN to get to the diagram. For instance #5
used on the e39...they are used on the e38 REAR
electric seats and/or head rests: