No real market
I used to manufacture a similar product in 2004. Ours were made from recycled mobile phone parts and a custom build of nokiaOS. We had two form factors, a CD-changer for the boot of you car and a shaver for your luggage. Our GPS & GSM aerials worked when the devices were in place no problem. They were very simple. Send it a txt msg and it sent you back the GPS coordinates. Being old skool devices they went days if not weeks without a charge.
When we found it hard to sell these, we tried offering recycled phones which could plug into your heating system so you could switch your heating on/off with a txt msg. Phones which could connect to your home alarm system so you got a txt msg when your alarm went off. Even alarms with cameras which would send you a picture message of the intruded zone.
However even with our bare materials being practically free we couldn't turn a profit enough to pay the salaries of two engineers and closed in 2005.
It's almost impossible to break into this market.