They are cooking.  You say it's an unheated garage?  What's the ambient temp outside?  You don't need both the plate and the heat lamp.  They don't need to be kept super warm, THEY NEED TO BE ABLE TO SELF REGULATE.  More chicks die from being too hot than too cold.  Once more for the people in the back.
I would have that heat lamp pointed downwards 

 so there is an obvious warm spot, and an obvious cool side.  I would put two bitty thermometers in there, one under the light where its hottest, one on the opposite where it's coolest.  And have a third for the ambient temp in the garage / basement whatever.  The hot side needs to be warm enough based on the week they are, but the cool side can be much, much cooler.  They can always pile up if they need more heat, but IF THERE IS NOWHERE TO ESCAPE THE HEAT, THEY WILL DIE.
Alternatively you can try the brooder plate only, but then you need to remember to raise the height up each week as they get taller, so they still fit under.  I've never used one.
Maybe try tweaking a few things, see what your thermometers are saying, and post updated pics.  I think a lot of people can learn from this right now.