There's a whole How-To Hopup Tracer in Arnies (I think). They had a huge night game where tracers were mandatory, lots of guys were modding their hopup...don't know how it went. Or maybe airsoftmechanics.com (I think that's where I found the thread, Killbuckets thread, that linked to another writeup). Might have been one of the airsoft forums in the US though.
Doesn't have to be clear...they drilled holes in the hopup unit. Step by step instructions, photos and very clear descriptions. Complete with wiring right to your battery pack or to an alternate power source.
I'll take a peek and see if I can find it again.
***edit***
That was quick to find.
Airsoft Mechanics - Tracer Unit Mod
http://forums.airsoftmechanics.com/i...p?topic=1903.0
couple of posts into the thread
1-st swords tutorial
http://1st-sword.com/forum/viewtopic.php?f=22&t=163
Tys