Plenum, intercooler, new exhaust, turbo wastegate actuator and flapper enlargement, injectors, valve springs, fuel pump to name a few bits.

Also, water is getting under the spare tyre panel, which is located under the middle of the boot.

Has the vehicle been in a rear ended accident at some point If so, the accident could have stressed the panel joints sealant and causing them to leak.

Water has a way of working its way into the car and not from very obvious places.

Also, check behind the tail lamp on that side to ensure the wiring loom rubber boot and seals are secure.

Forgot to ask, is it a sedan or station wagon If wagon then could be from the wiring loom rubber boot at the top by the hinges or the rear washer bottle supply linenozzle.

Strip out the spare wheel, tools, boot mat, rear panel cover, etc, so its all bare panels and get some one to lie in the boot with a torch, close the boot and run the hose over the rear of the car while the person in the boot has a good look inside.

Jump in the boot with a torch and have someone run the hose indirectly over the boot - no direct jets of water.

Other than that Id say its either a defective boot seal rubber or the body flange the boot seal rubber mounts onto is compromised by a rust spot letting water through I have removed the boot seal rubber before and smeared RTV sealant around the boot seal flange and into the mount grove on the boot seal rubber then refitted the seal rubber back onto the flange (clean up any excess RTV before it sets) Worked well for a persistant leak I had in a Cressida years ago, never leaked again after that excercise.

Check the rear window to see if its seal isnt compromised.it can happen and its possible too for rust to form in the window glass aperture and let water in that way.

On a good dry day get a good torch,climb into boot and have a helper mist spray water over the rear areas of the car and youll no doubt locate the leak.

Make sure you dry out the boot beforehand to positively identify where leak presents itself.

Must be a fairly bad leak if theres a quantity of water pooling in the tyre well, good luck.

The earlier Toyota products though like RX60 -GX60 GX71MX73 Cressidas had the tail lights mount in an open type aperture in the body and these types of mountings were very prone to leakage and dust ingress past the factory seal gasket, these gasket seals between the open type body apertures and lamp units were rather prone to deterioration and rust around the apertures was often a problem as well.

The Cressidas I had, the first thing I did with all of them ( I had a total of 8 of them over 14 years.) was to remove the tail lamp units and seal the apertures with RTV Instant Gasket and make sure they had a continuous strip along the top half of the lamp to fend off any likely water entry if water was present The sealant could be neatly done to look like a factory finish item, worked perfectly for me.

I also removed the boot seal rubbers on all my Cressidas and used RTV sealant and replaced the rubber boot seal on its flange as described in post 10, never had a problem with any of them once I did that, worked like a charm.

I have used an eyelet punch on those drain plugs to punch a small hole in the plugs odd times just enough to let water out.

The Turbo XR6 has a damn good engine for sure, is yours the Herrod version Have you had any issues with rust at all, especially leaks in the rear window area and under rear parcel shelf regions where they are prone Ford Australia had a lot of paint issues with Blue color Falcons for some reason and there has been reference to the subject on here before about it I believe Lots of Falcons and Commodores for that matter have rather anemic paint jobs where there are areas that havent had paint AT ALL and are still bare metal, absolutely shoddy workmanship to say the least as has also been referred to on here previously and I have personally seen first hand disgusting is the only way of describing what I saw recently.

Also spray round the inside of the rear guards.

Good luck. Does it get wet when parked at different angles.

The only interior exterior link is the wiring through a body grommet in the lamp unit recess in the body which its highly unlikely any water will pass through unless its totally immersed or heavily hosed in that area.

Defective boot seal or defective seal at rear screen is most likely cause in the Camry, also body joins seal lines need to be checked for possible seal defect A rust pin hole will let in a surprising amount of water in a short time and boot seal flange area and rear screen aperture needs to be checked for small rust holes that could possibly be lurking hidden under boot seal on the boot seal flange.

I own a Toyota Avalon and the method of tail lamp unit mounting is virtually the same as I describe in the Camry.

Avalon is pretty much part of the Camry family models anyway.

How To Seal Car Boot