Somebody on here said they went to the dealership and had them hook up a device (I can't recall the name of it at the moment) to reset the ride height. He needed the rear lowered so they plugged the device in to reset the right height, had some guys sit in the trunk, and then set the height. From what he said, it worked out and was fairly quick and easy. I've been looking to get the front end lowered on mine because it looks uneven compared to the rear, but haven't called around to the local dealers to see if they can do this.
The adjustable air suspension route is available, but you're looking at thousands as opposed to free or a couple hundred with the way I mentioned above.
Rear height is monitored and air is pumped into/bled from the shock auxiliary bladders to compensate for extra weight in the trunk or back seat. It is NOT "height control" as such, but there is a small, limited range of adjustment to allow for about 0.5" of rear height, unloaded, adjustment.
Even back in the Seville/Deville cars, the active suspension models seem to carry a slight amount of front rake. The GM Tech-II "adjustment" is called "bump stop setting". I believe the member posting about assistants sitting on the trunk lip was referring to a method of actually observing if the ELC system is working.......... You weigh down the rear end and the system should return it to the same level as unloaded - no overall height change.
IF front height can be electronically adjusted in the later "DTS", what is the method for compressing the springs without coil cutting ?