taking 12v and knocking it down to 5

I have an amp that has an internal fan. At 60*C, the fan turns on and gets 7V. At 80*C, the fan gets the full voltage of the car. What I'm trying to do: I have 4 computer case fans and I'm trying to get them to come on as the amps get hotter. Instead of doing a thermistor circuit, I want to try something a little easier. I want the fans to come on slowly at a lower voltage, like the 7V that the internal fan does, but how would I accomplish that? I will be tying on to the existing fan, so I can get full voltage to the fans when the amp hits 80*, but is there a way I can have them turn on to around 7V when the internal fan hits 7V. I cant think of anything off hand, so heres one of my ideas: Is there an inexpensive way to knock down 12V to around 5? I would hook this up to an ignition wire so the fan will turn on when the amps do and take that wire and knock it down to 5V. If anyone else has any other suggestions, Im certainly open to them! -Cody

best buy sales a piece that would work its called HITCHIKER it sales for $20 bucks hook one end to 12volts the other end to your fans and u can controll the voltage by turning a dial on the unit ...my friends call me butcher i fear no wires that dont belong.......................