When the key is turned to the start position it is normal to loose
electrical power to certain accessories because all the voltage and
amperage is being diverted to the starter to engage the engine. Does
the engine start or try to start when you turn the key? Is that the
problem? If you have a dead key when turned to start it could be a
low battery, but you said you checked that. It could also be the
ignition switch inside the steering column, not the key lock into which
you insert the key. A multi-meter and wiring diagram is going to be
required. With someone turning the key to the start position I would
first check the solenoid relay mounted on the fender well near the
battery making sure it is working and sending voltage to the starter.