Trunk lid on 2003 cadillac cts is hard to raise

On page 2-16 in the owners manual there is no mention of using a key to access the trunk. The latch and lock are electric.
You can access the trunk space thru the back seat in an emergency.

Actuator in trunk disconnected or Brocken

Check to see if there is a recall for your vehicle on this or any problem. Probably your lid isn't set properly, and just needs adjusting, or the seal was damaged.

Replace or regass the strutts

look in fuse panel find window relay and or fuse replace. then disconect battery for 30 secs to reset computer.

You go to a dealers parts counter
or the auto parts store

Let them do all the guessing and measuring etc

Try a little taller tire instead. Blocking the suspension certainly isn't going to improve the way it goes down the road, and it may cause early failure of suspension components, uneven tire wear, poor handling.. long list.

lp

You have small struts that hold the trunk open and they do wear out over years of use, but are especially prone to not doing their job when the temperature is very cold. Those struts are not too expensive ($20-$25) and are usually pretty easy to replace.

check the location of each spring. Make one side tighter by one notch.

Move the end of the luggage compartment lid torsion bar to a higher position.

sounds like the the trunk pull down assy is not raising. On the pull down is a black little piece of plastic ( part of the switch ), Flip up plastic to see if motor wll raise pull down, then the trunk can be closed normally. If it doesn't raise, further diagnosis will be required in the pull down motor area