adding a sub to toyota stereo

hi there i would like to hook up a sub and amp to my 2003 Rav 4"s stock deck.

i'm thinkin about using speaker to line level converters, but i'm not sure if i can just tap into the speaker leads. could sure use some advice

Find yourself a good LOC and do indeed tap into the speaker leads.  A good LOC will handle a ton of input power (some factory systems have hidden amplifiers) and will also have a gain adjustment on them. Scosche makes a decent one. www.scosche.com

thanks forbidden.  Heres a little diagram of my plan.