I will repair your Reatta (and many other GM power antenna) with a new steel cable, clean and lubricate for $65 including return shipping.
Most power antenna stop going up/down because of a broken plastic cord that is attached to the top section of the mast. This breaks and GM has discontinued the part. Other vendors now want around $35 plus shipping just for the replacement plastic part.
I developed a method of using plastic coated steel wire in place of the plastic and have repaired over 20 using this method. This steel wire should be a lifetime fix. I will replace any that break while in the original owners hands.