Welcome to the site..
Door ajar switch is mounted upside down in the door latch.. They like to 'hang' or stick from debris collecting on the pin.. Sometimes spraying the door latch with some silcone or graphite based lube in that latch area and then repeatedly opening/closing the door will get it to function..
If you open the drivers door, does the interior/dome lights come on? Any 'door ajar' indication on the IP panel?
If none, you could always remove the door panel (very easy task) peel back the moisture guard (towards the back/latch) and reach in and unplug the switch as a test..
05 F250 C/C FX4 6.0
1969 Firebird Trans Am
2003 Hayabusa GSXR1300