James and I have both done it on our VW's.
It usually takes a few light coats to get a tint on. Tailights should be extremely clean, and a light sanding helps (so does shaving off any raised writing on the tailights with a razor blade)
And clear coat after for a proper shine. Both James and mine are half tints. Just turning the orange part of the lamp smoke.
Stock
Clear
Smoked (done on the orange ones)
James Corrado
Stock lamps look like this.
Here's a Civic with a full tinted tail.