99 Malibu LS Check engine & brake light issues

I have a 99 Malibu LS (Automatic, V6), with approx. 76,000 miles on it. Last month my brake & ABS light began flashing intermittently. I got a complete rear brake job done, this fixed the problem for about 2 weeks, then the light flashed again. The check engine light also began coming on. The mechanic who did my brake job ran a diagnostic on the check engine light and wanted to replace the fuel tank vent valve, the evaporation control canister, and two sensors. The lights were inconsistently coming on, but the car continued to run normally: no rough idling, no brake problems, no increase in temperature. I eventually took it to another mechanic for a second opinion who tightened a loose bolt on one rear brake that was allowing brake fluid to leak (hopefully fixing the brake light problem), and replaced my ignition switch. They claimed the computer had been throwing codes P1665 & P1676, which they said have to do with the Aux controls. Was I fleeced, or should replacing the ignition switch (an expensive part) take care of the problem? This repair was done just this week.
Thanks so much for any input.