New OAT for old Arrow

Hi!

Anyone have any experience in changing OAT gauge an old Cherokees?
Ours has the old probe sticking out the tope of the windshield, but after
replacing it twice with new units, it still doesn't work.....

We would love to get a newer more reliable digital OAT readout.

Anyone got any ideas of a good unit, preferably with a website.
Is there a lot of fuss to change this paper wise?
I mean, do we need an approval, or is this not considered a major change?

I saw a combined OAT, carbon monoxide alarm, clock, stopwatch etc instrument
in an add in AOPA pilot magazine.
Anyone got one of these installed in a Cherokee?

Thanks for any tips,

Frode Berg
Arrow: LN-LMR