I have a 97 2500 V10 4x4 and my radiator leaks up at the top drivers side corner just to the right of where the top hose connects to the radiator.
The reason your leak (like mine) is happening randomly is that once yur coolant level drops below the leak your engine will still stay cool and you will only notice a problem if you park sideways on a hillside with the drivers side of the truck at the lowesr point allowing more coolant to leak out.
These radiators seem to be weak in this area. Also, my 97's underhood configuration is just like how you describe your 99.
How I stopped my leak is with a good bottle of stop leak. You'll have to give this time to work so keep some water handy and check your coolant level often
Hope this helps.