Sorry to start another thread but I'm desperate to get this fixed fast as it's really costing me not having this vehicle on the road now.

Got a '11 Crown Vic that's been overheating because the rad fan hasn't been kicking on when it should. I replaced the fan and fan controller with a new unit and it's still not working right. Sometimes the fan kicks on when I turn the a/c on, and other times it doesn't. Sometimes it turns on when the engine gets hot and sometimes it doesn't.

Water pump is known to be good, flushed system both ways and the thermostat is new.

Any ideas?

 

To better help you the DTC's if any in regards to weather the PCM has detected an over temp condition for ECT/CHT would be a big help and starting point to move forward.

If the fan runs within and continuous A/C command, it may point to a failing CHT sensor and the PCM not seeing the over temp condition.
If the gauge follows the over temp accurately (limp mode may be active if hot enough) and manual checks with a probe or gun, then the CHT may be accurate in its reporting.

The PCM controls the fans variable speed by supplying a negative duty cycle generally 10-90% when active (achieving 30-100% speed) on the FCV (Fan control variable) input circuit into the fan module. If you had access to any codes,, or better yet into the system PIDS you could verify these values, command function etc. Depending on what diagnostic tool you have, or access to, self test/function, and code retrieval is possible
If you can get the DLC active would be a good start, and do you have a diagnostic tool?
At a guess, I'd suspect the PCM control is intermittent (could be harness/connector issues) or drops out (self test would help here) possibly a CHT concern but A/C HI should run the fan.
Being you stated its an ex law vehicle and the wiring has been chopped, more the need to do some checks...