I bought a 1996 Mercury Grand Marquee I was wanting to know is the reservoir used for antifreeze back up.?

There are two lines in the reservoir. Max and min. Get some premix coolant at a car store and put some of the coolant between max and min, halfway is good. Check the level again after a week. You service as required again. It should not require servicing again after the second or third time. If you have to service the reservoir everytime, you need to see a mechanic.

Short answer - The expansion tank should contain the same coolant as in the radiator.

Long answer if you're interested - When coolant in the engine and radiator gets hot, it expands and the cooling system becomes pressurised. This is good in small doses because water under pressure has a higher boiling point than if it were at atmospheric pressure. This superheated coolant (temperature above boiling but still liquid) is still able to cool the engine which is running at a couple of hundred degrees - there is a heat gradient. The reason they bother pressurising the cooling system instead of leaving it at atmospheric is because a cooling system with a larger heat capacity needs less coolant and therefore smaller radiators which all helps with packaging under the hood. However there is a risk the pressure in the system may get too high. If the cooling system gets over pressurised, you run the risk of something breaking (think blown/burst hoses, burst radiators and other similar nastiness). The pressure in the cooling system therefore has to be controlled. This is done by the pressure cap on top of the radiator. This is a spring loaded cap. When the cooling system pressure gets beyond a pre-set point. it overcomes the spring pressure of the cap and excess coolant is vented out of the radiator through a small tube at the side of the filler neck. Back in the day, this tube only had a hose on it which directed excess coolant to the ground. This meant that when the engine cooled back down, the coolant contracted and created a vacuum in the radiator. This would just suck in air. This continual loss of coolant meant the radiator had to be topped up very regularly. Then someone realised that dropping random amounts of ethylene glycol all over the road was a bit naughty. Ethylene Glycol in coolant is a very toxic poison and also exceptionally slippery, so it's not the kind of thing you want on the road and washing into your waterways. So an expansion tank was attached to the radiator breather tube. What happens now is that the excess coolant from the radiator is caught in the expansion tank instead of being dumped. When the coolant contracts, the partial vacuum in the radiator sucks it back into the radiator, maintaining the correct level at all times. You need a little coolant in there so that the radiator doesn't suck back air.

it should have the same content as what is in the radiator

it is not a backup at all but a reservoir for water when it expands from heat

It is supposed to be partially full, yes. There are lines on it indicating the proper level when cold and when hot. If empty, fill with proper mixture of the AF that goes in the car.