2004 and 2006 Trailblazer Electrical Problems

I have two Trailblazers. The 2004 has 132000 miles, 90% of which is interstate miles. On this vehicle, in the last two months, I have lost fan speeds 1, 2 and 5 on the front fan control, the cruise control works intermittenly and the tachometer at times is off 2000 RPM, starting at 2000 instead of 0.

On the 2006 Trailblazer with 43000 miles, the gas guage changes constantly and never reads correctly, the radio cuts on and off and I have lost fan speed number 5. All of the problems just started this week.

Judging by the problems on these two vehicles, I am assuming that there may be some type of electrical issue causing all of this. Aside from two trips to the dealer and a day off of work, could you suggest a starting point to find the problems.

Thanks.

Brad