I've only had a portion of mine temporarily lowered and it was surprisingly easy, there are very few fasteners, but a ton of tucking and "friction mounting". Requires a lot of interior A, B, C, and D trim panel to be pulled loose. It can be more involved if you have the rear-seat entertainment center, a power moonroof, and/or Sirius antenna.
Best to follow the well-written instructions found here:
EBSCOhost Login
username is rrcc
password is rebsco
Click "Auto Repair Reference Center"
Add your year/make/model
Next page you'll select "Repair Info"
In the search box type Headliner
In those results, 'removal and installation'
An educational trip to the local u-pull-it yard might be worthwhile.
If dropping the headliner is too involved, I'd probably look into running a whole new rubber line along the channel in the lower door sills, installed just like you were wiring in a huge speaker amplifier to the car battery, up thru the firewall and to the washer pump.