It wasn't that difficult, but then again it's a bit of a half-assed job... Here's how I did it and prolly isn't the best way. Wasn't expensive though.
You need a clear panel for this
[*]get the thing off (4 nuts)
[*]remove any silver paint from letters so they are clear
[*]cut the back out in the shape of a letter box (you'll never get it open as it's sealed with goo, grrrrrrrr!)
[*]get some translucent red self adhesive vinyl and stick to inside of panel (mine needed 2 thicknesses to get the colour right) - £free
[*]clean it well and carefully mask letters and black bits (masking the letters took a razor sharp knife and a lot of time)
[*]get (right colour) spray paint and plastic primer - £12
[*]prime, spray to a good thickness
[*]get a neon tube, mine's for a PC case but is a bit short, and fix inside - I used a hot-glue gun near the ends. - £13
[*]test
[*]seal up with whatever you like
[*]tag transformer wires onto tail light circuit
[*]fix it back on
[*]connect up
[*]grin! - £priceless!
[/list=1]
Hardest bits were masking the letters and getting the right colour paint - the QM1 I had mixed was no where near the QM1 the car's sprayed in (apparently there are at least 2 QM1s...).
Cost just over 25 quid ($65 ish). Bargain.