I don't know from personal experience if there is an existing hole in the
firewall of a 2002 Chevy Blazer that the power wire can be routed through.
Perhaps someone else can tell you.

But in case there is no easy way, you may have to drill your own hole (which
I have had to do many times myself).  The trick is to find a spot, usually
up high,  where you can reach up underneath the dash and touch the firewall
(or at least a place very close to the firewall).  Then drill your hole from
the engine compartment at that approximate place (you may have some
insulation comming out of the hole).  It's best to use rubber gromets around
the metal holes you will make to protect the wire, but since the large guage
power wire I am generally using (usually 4 or 2 guage)  is nearly as large
as the diameter of the hole itself, I sometimes just wrap the power cable up
with multiple layers of electrical tape at the place it passes through the
hole.  This protects the wire just fine and usually ensures a nice tight fit
as well.  Sometimes the firewalls are two layers of metal with insulation in
between.  This is OK as long as you drill the hole nice and straight through
both sheets of metal and then insert stiff cable (anything thicker than 8
gauge is usually pretty stiff) through both sheets.

It really isn't that difficult to do if you cannot locate an existing hole.

Good luck,

MOSFET