Rapid acceleration 2008 Mercury Grand Marquis. Yesterday my boss's wife was in the Merc, it self-accelerated and crashed into a fence. Does anyone know of an issue with this make concerning rapid acceleration or the electronic throttle control?

Defective head unit?

You have not shared what art of the country you live in, but I may be able to guess your answer.
All cars as old as yours are subject to brake failure, usually from leaking or rusted components.
A good inspection now should show you the rear brakes are bad, and perhaps the brake lines have rusted through.
Could also be a rear brake cylinder has failed, leaking out your brake fluid, and causing the brakes to give out when you needed them most.
Routine inspection probably would not have caught that, but having your brakes serviced every 7 - 10 years should have prevented it.
Lots of people do not do a complete brake service because it can get really expensive, though it would prevent your accident.
The acceleration problem is a whole different matter.
God bless your efforts.

Hi, The link below is a youtube video showing a possible solution to your problem, Also... Down the right side of the link are more video solutions.....CHEERS..

Is Your Car Hesitating It May Need New Throttle Position Sensor

Sounds like a vacuum leak Check all of the vacuum connections

Its sounding like it could have your timing off...either electrical, or internally. If all plugs have good spark, scratch electrical. Then you could be looking at a timing chain.

Is it VERY quiet at the tail pipe?Could be the converter on the exhaust is plugged up.

maps sensor

sounds like a vacuum problem. Check vacuum lines. Maybe hooked up wrong.

idle control valve is bad. buy one from dealer, about $110, eazy to replace

A full ignition tune up would be new spark plugs and wires.A cap and rotor if it has those. Do any fuel system cleaning before changing the plugs.