WEST PEORIA — In need of help, Santa Claus called Eric Sparks. At least, that's the story from Sparks, whose front yard sports a custom-made "Letters to Santa" mailbox. Children slide letters to St. Nick into the box, and days later they'll get a personalized reply in the mail. Sparks, keen on maintaining the true-to-Santa storyline to kids, insists he has just a passing role in the process. "Since Santa knows I live on a busy corner, he asked if he could use my yard," Sparks, 36, says matter-of-factly. For five years, the third-shift nurse has lived at 327 N. Kickapoo Terrace, at the corner of traffic heavy Sterling Avenue. Three years ago, the Santa-letters idea sprang from a scary Christmastime when his father, Tom Sparks, suffered a serious heart attack.