Rain water builds up in the right rear footwell,how do i find the source and fix it?

fresh water there has only 2 sources.
1: rain (or parked near powerful sprinklers)
2: condensate from HVAC Evaporator pan drain clogged
are you using AC cooling? yes/no? or heater mode,in defrost ,mode ,yes/no? ,if NO and NO, see #1

rain gets from bad door gaskets(seals), or broken glass or from
front top , front of windshield glass ,cowling drains clogged.

take it back tell them they failed.

Once you have checked it isn't a car window that is slightly open there is three main reason for water in the foot wells. Sun roof, door seals and heater matrix.
If you have a sun roof fitted, check it is closed correctly. Open it all the way and check the running grooves it sits in for dirt and blockages. There are drain holes in the runners so try cleaning the runner grooves with an old toothbrush.
Open the front door on that side of the vehicle. Take a good look at the rubber seal the door presses against when it is closed. Any damage or splits can create leaks into the vehicle. Check the rear door seal for the same thing. If you can, check all the other door seals and the trunk/boot seal as well. These seals are usually a tight fitting 'push on' type which you can replace yourself.
This will sound silly but does the 'water' smell of anything? The heater matrix is under dashboard and is what supplies the heat to the heater blower controls on the dashboard. If this is leaking the leak will have a sweet smell as this is coolant from the engine. This is a big repair as you often have to remove the whole dash to repair it.

Check where the ECU is mounted as I have an MX-5 & the ECU is mounted in the passenger side footwell & when it was raining the soft-top was letting water in at the rear & it was collecting in the footwell & affecting the ECU but only when the weather was wet. Worth a look?

I have had water intermittently in the right rear trunk area (below the battery) every since an rear end accident. The dealer tells me that there is a vent back there that gets clogged up or opened up and lets water get in there. Not sure that I believe that, but that's the best explanation that they could give me.

got a sunroof in this? If so, clean the drain tubes.

try underneath car normally plastic drain plugs ,if one missing water can get in that way

does this happen only after it rains?
check under the wheel well or near by for rust the other place maybe around the rear window seals leaking in under them.

I had the same problem on my 2003 Audi.The guy who fixed it told me that he sees alot of Passats with this problem.
The drainage holes underneath the battery get blocked by leaves etc. leading to a build up of water.This then leaks through the heating ducts into the rear footwell.
In my case it wiped out the electronics for the alarm and central locking(£200 for a new one).
Hope this helps you.

The 1997 Sebring convertible has rocker panels (the metal that goes from front to back wheel well and directly under the doors) that have five drain channels each.  The sloshing noise occurs when the drain holes plug up and a rocker panel fills up with water.  Often, the water will then leak into the right rear footwell, soaking the carpet.  A quick fix is to locate the drain holes and run a flat (unused) twistem-tie (they come with your trash bags) through each hole several times.  Commonly, doing so will result in a deluge of water, along with spider web parts, leaf parts, and whatever else washed into the rocker panel fromo the rear window.

To find the drain holes, just 1) lie under the car with a flashlight, or 2) sit alongside the car with one of those mirrors that come on a telescoping stick.  Since I don't like to lie in dirty water, I use the mirror to guide my twistem-tie (or ice pick or spring wire or whatever) into each hole.  NOTE:  the drain holes are flat and run at a 30 degree angle from back to front on the right side and from front to back on the left side.