On*Star and his cell phone could provide the same info couldn’t they?
Yes, and there was controversy around eight or so months ago about On*Star working with police.
OnStar uses a GPS and a cellular connection. Getting cell phone coordinates varies. Most phones allow a privacy setting that only allows phone location in a 911 call. You can set the privacy level lower to allow getting a fix at any time if you want to use "location based" services. The cellular phone module I'll be using on my rail cars is so sensitive that it can be tracked when wrapped in a metal mesh bank bag, stashed in a trunk and parked in a concrete building. That takes more than simple GPS. It requires A-GPS and/or A-FLT (assisted forward link trilateration) or TDOA (time difference of arrival) to compensate for not having a direct view of the sky. TDOA can accomplish an 100 ft CEP by itself. Plenty good enough for what the cops needed in this instance. TDOA is essentially a high resolution "ping" of the handset by the tower. That establishes a circular distance from the tower. Do that with 3 towers and the intersection of the circles yields a fix.
Suffice to say, there are many ways to track a device. It's good that this case did the job. As for the 4th amendement, many ex-cons sign a 4th amendment waiver. If that was on file for this individual, the 4th amendment issue doesn't apply.