keeping the temperature up in the winter

I have my new ball in a 35qt steralite tub with a UTH on one side.
on the side that his hide is on belly heat is about 90degrees and ambient is 85.
but on the cooler side the temps are 70/73.:confused:
should i add another heat pad?
im using this http://www.reptilebasics.com/RBI-201-p-16245.html to check the temps routinely throughout the day.

Its probably a good idea to get a second smaller UTH for the cool side. Before I started using racks and just had tubs out in the open I needed to supplement the cool side in the winter time as well. 78 is the lowest cool side temps I'm personally comfortable with.

I agree with Spaniard -- you can get a cheap heat mat and rheostat for under $25. Stick it on the other side, and run it all the way at low if necessary.

Alternately, you could buy a room heater and bump the ambient air temp up a few degrees, and just turn your hotside UTH down (if that would even be necessary). Oil-filled heaters are under $40 at Walmart or Home Depot, and they are safe to run 24/7 as far as I know.