with a PoD within the eighteenth century, make it so that the Spanish retain control of at least one colony in South America to at until at least 1900. The bigger the colony (or more colonies) the better.
Honestly, the 1800s is fine too, I just didn't know if that'd be too hardPOD: The Spanish 1820 rebellion doesn't happen, the Spanish army overwhelms Colombian forces. While low level fighting continues for years. Bolivar can't attack Peru from the north, the Chilenean forces San Martin leads from the south are defeated by loyalist forces in Peru, as the loyalists aren't facing a two front war.
Peru remains loyalist, Colombia is a toss up. Chile, Argentina, Paraguay and Uruguay might remain independent if the Spanish are bogged down in Colombia, but Brazil might end up annexing Uruguay as Argentina might not feel comfortable in getting in a war with Brazil with Spanish armies still operating in the Upper Peru.
EDIT: Oops, checked you wanted a POD in the 18th century, not the 19th
As in kill Riego (b.1785) and this particular army won't head for Madrid rather than embarking for South America. Kill Ferdinand VII (b.1784) and there won't be reason for a rebellion in the first place.All juan would have to do is stick the POD for the 1820 non-rebellion back in the 18th century, and there you go. The action would still all take place in the 19th century.