pansa,
I had an Infinity basslink in my old 01 RAV4 and I don't think it would fit in the stock sub location in the 03 4runner. I've taken off the rear side panels of my 03 4runner (w/ JBL) and noticed there's not much room for a sub of that size, especially since the basslink is 10" and the stock sub is 8". Also, if you were to stuff it into the stock location you wouldn't be able to adjust the various settings on the basslink (gain, crossover freq, bass boost).