the fuse for the dome light blows if the fuse does not blow the battery discharges
dome lights and map lights fuse blows on 2010 Mercury Grand Marquis
Which fuse are you referring to, what number and which fuse box? Since you don't answer, there is a problem with the lighting control module if your battery is running down and the lights are the problem. The lighting control module will shut the lighting circuits off after a short period of time.
passanger compartment fuse panel fuse 18
That's the one that goes straight to the lighting control module, it feeds the on demand lighting. Includes glove box, trunk, vanity lights. The control module is supposed to disconnect the power to all lights in a short period of time, around 5 minutes. If the battery is going dead it has a problem between the fuse and the light control module. The Lighting control module should not be discounted, because they do go bad as well, but usually has different symptoms.
how do I find the short between the interior lights and fuse 18
Do you have any electrical tools, meters. I would meter the draw on the circuit to verify where the draw is, and also verify the control module is timing out.