Several tour companies in Trujillo [1] cover the city, Huaca de la Luna [2] y Sol, Chan Chan [3], and even El Brujo [4]. The best of these are Guia Tours (Independencia 580, tel. 044/23-4856, guiatour [at] amauta [dot] rcp [dot] net [dot] pe, US$8–12) and Colonial Tours (Independencia 616, tel. 044/25-8261, daily, US$5–8), which is based out of Hostal Colonial.
If you prefer a personal tour, Benito Jáuregui Rosas (Pizarro 318, tel. 044/957-6409, guiabenitojauregui [at] yahoo [dot] es) gives thorough tours in Spanish; Carmen Linares Santos (tel. 044/930-7177, karmelinsa [at] hotmail [dot] com) is fluent in English; and Michael White (Cahuide 495, tel. 044/966-2710, casadeclara [at] yahoo [dot] com) tackles both of those languages, plus French, Italian, and German.
Links:
[1] http://www.moon.com/destinations/peru/trujillo-and-the-north-coast/trujillo
[2] http://www.moon.com/destinations/peru/trujillo-and-the-north-coast/trujillo/sights/huaca-de-la-luna
[3] http://www.moon.com/destinations/peru/trujillo-and-the-north-coast/trujillo/sights/chan-chan
[4] http://www.moon.com/destinations/peru/trujillo-and-the-north-coast/trujillo/sights/el-brujo