RX300-Location of tail-light wiring.

Hello,

I have a 2000 RX300 that I wish to install trailer wiring. The original window sticker says "towing prep package" but evidently that doesn't mean that this thing is pre-wired. I checked under the plastic box under the gas tank and there is no plug. Where would be the best place to tap into in order to install a converter. Does the wiring come past the spare tire in a bundle??

Thanks in advance. Esch