Had diagnostic run--mechanic showed me P0500--which is speed sensor--which he said traced to bad speedometer--new one cost ^$200 for part alone. P0600 AT communication--he said "possibly" related to other problem. Experienced blinking cruise light which went solid & cruise function failed then noticed SES light on steady--traveling around 60 mph at the time. At approx. 1,000 and 1500 miles earlier, per previous owner, his dealership had only mentioned P0500 showing on two previous DTC. Mechanic told me if speedometer not replaced, this could lead to transmission malfunctions down the road. 97 Max SE--should I consider used speedometer or even investigate cause further? Any comments, info., or suggestions very much appreciated asap. TIA