A lot of bazooka bass tubes need little power, I have heard many people not using an amp with them with acceptable results. There really isn't a way to easily low pass it without an amp (remove the high sounds that will distort your sub), you could get an EQ for like $25 but if you have a budget of $0 forget it, you'll get noticeable results either way.
What you could do, which I would recommend, is taking out your head unit, connecting your front and rear speakers both to the front right & left wires, and running a new wire to your bass tube that is connected to your right and left rear speaker outs. This way you could balance the power by using the fader function on your head unit. You could easily send 75% of the signal to the tube (rear channels) and 20% to the rest of the speakers (front channel).
There is probably a wiring diagram on the top of your head unit, but if there isn't one, this should help -
www.installdr.com/Harnesses/Toyota-Wiring.pdf
Post back if you end up doing it, it should turn out alright.