I have the same problem. Mine is leaking where the upper hose meets the neck of the radiator. I took it off and found that there was a crack 1/4 to 1/3 of the way around the neck at it's base. It is possible that when your mechanic was replacing your hoses, he put too much pressure on the neck and cracked it. (I think that is what happened to mine) I would suggest pulling the hose an checking for cracks. If it turns out to be cracked, there is something called "plastic tank repair" that I picked up at auto Zone. It solved the problem for about 24 hours and then started leaking again, but it slowed it down a tremendous amount so that I don't have to worry about losing too much.

If it is cracked, you will have to get a new one. Unfortunately, that is the reality of plastic radiators. If replacement is a requirement, I suggest ordering an all metal radiator. RPR has them. (http://www.rprusa.com/) They are only a few bucks more than the plastic ones and can be repaired if something goes wrong with them.

Hope that helped a little.

Sam