i was wondering what is the best double side tape brand to put on add on kit...
because my back kit is add on and also side skirts.

Is it lip kits or full bumpers?? I wouldnt suggest tape I would suggest a couple screws. The side skirts are extremely easy and you basically just have to put some screws under your door sill so you wont notice it at all.

You will need to rivet/screw still. But the tape you want for the job is 3M Double Sided Tape. Holds like all hell.

i know that skrew under neath the door panel would help. but i really want the double side tape that the dealer have on there oem body kits.

3M double sided tape can be found at a hardware store.

for the side skirts 2 screws on the door sill and 3M tape one for the front middle and rear.

the front bumper i recommend screws and not tape same goes for the rear.

you might wana think about how you want it to handle in a crash, or bumping a kerb, with screws it will crack, with tape its more likely to bend.

basically it depends on what your installing. I've put body kits on, and have seen shops use tape in car magazines. If it's a lip kit or extensions or a spoiler... yea I might use double sided tape. But a bumper... no. I'm not saying it won't work, but I can't say it would work either. In those situations, it's best to go with the tried and true methods.