Just got my General ticket and want to mount my htx-10 in the car. I have
a
Saturn coupe and want to mount a 10m hamstick on the trunk and also want
to
install an amp. This needs to be a cheap evolution and am unsure about
amplifiers and the installation. I only need to push 100 watts I guess.
The
htx is 25 watts ssb. Will I need an extra battery if I run an amp? Or do
I
need an amp at all?

Going from 25 to 100 watts will not be that much of a gain. Only about 6 db.
That is what some think of as one S unit. Try it without the amp first and
see how well you do compaired to others. Instead of an amp you may want to
just look for a used 100 watt all band rig. The battery in the car is fine.
YOU do not need anotehr battery as long as the car is running. With the car
off much transmitting and you will need a seperate battery to get you
started again. On receive you can leave it on for several hours without any
problems.