Well, it seems at this point that the airplane was in the correct air/ground mode. The front (and back) wheel switches were (likely) in the correct logic state.
It was just one out of dozens of relays that failed to act correctly upon this air/ground signal, affecting ultimately only two components out of more than a hundred that depend on ground/air activation: the RAT probe heater and the TOWS (the other two systems wired to this relay being redundantly serviced from other, working relays).
If you put a couple of indicators in the cockpit wired to the ground sensors ... the airplane would've correctly indicated it was on the ground.
The change from ground to air mode was even signaled to the DFR correctly, as all the relays depending on it worked except for one (it seems).
I think the MD-82 "works", as we have seen the reasonable safety record considering how old it is, but it is indeed a bit "underdesigned" when it came to the alarm for a "potential killer item". It has a single "common" point of failure that gives very little/no warning. Maintenance manuals must CLEARLY include that probe heater on the ground inmediately must suspect inop TOWS.
Boeing solved the problem the best and cheapest way possible though: don't ever TO w/o checking TOWS first soon before.
Making modifications to the plane also introduces new, unknown risks, so it's not as simple as throwing in a couple of diagnosis or status lights which, BTW, can also fail on their own or be overlooked. I'm not saying they shouldn't study some simple one that could increase the reliability of the system.
...and of course we're all assuming that there were no electrical modifications in the aircraft history. Given its previous owners, who knows?
The judge has requested the police to find the entire history of the airplane since the day it was manufactured, including any and all modifications done by previous owners.