When building a pinewood derby car and adding weights where should they be installed? front, rear, center?

in a groove hollowed out on the bottom of the car, between the front and rear axels

or, if the car is tall enough at the rear and you're using the right kind of weights (I used fishing sinkers), drill a hole as deeply as possible and stuff it with a little less than the needed amount of weight, then seal the hole with wood filler

It really depends on the design. I would recommend the top of the car, and put them in the front. That way, your car is pulled down the track and it runs evenly, not wobbly. However, if you have a really cool design, just put them wherever you feel looks good. It doesn't make a huge difference.

My first pinewood derby car had no weight and lost every race but it won 1st place for design:)