Is a automobile rider agreement legal with just signatures? the top of the rider agreement was not filled out at all. all are blanks not filled in b

It depends whether the omitted information was material, and, if so, whether that information can be gleaned from other parts of the contract or the parties' performance.  For example, the fact that the date of contracting was left blank is probably unimportant, the names of buyer and seller can be gleaned from the rest of the contract, but the specific car purchased is material and may not be mentioned in the rest of the contract.  Of course, if the buyer took delivery and didn't protest that the wrong car was delivered, that should not be an issue.