does anyone know how to make one?

and what if u just ran your factory deck speaker output directly into the RCA inputs of ur amp???

with every do-it-yourself circuit you could think of. It will show you the proper way to make a signal converter, hifi eqs and line drivers. It is a rather large table of contents linked to electronics sites galore....

speaker to RCA is nothing more than an isolation transformer and a resistor voltage divider.