The sources of supply of most leftist insurgencies in Central America in the 1980s--very particularly the FMLN in El Salvador--were complex. Salvadoran government forces backed by the U.S. reported finding many E. German MPiKM-72s, Yugoslav RPK automatic rifles, and a few, yes, North Korean Kalashnikovs. There is an old, oft-repeated story that the FMLN had U.S. M16s before the U.S.-backed Salvadorans... Supposedly U.S. weapons were transshipped from Ethiopia (where Cuban troops intervened vs. Somalia in 1977-1978) and Vietnam (captured ex-ARVN) and supplied in a few cases.
Somewhat oddly, the first really big supply of Kalashnikovs in Honduras came from Israeli captured weapons sold to the Contra counter-revolutionaries of Nicaragua...
Fidel tired of the Soviets almost immediately... Certainly he cost the USSR a great deal in terms of subsidized trade and so on. When Ronald Reagan became U.S. President and Alexander Haig was vowing to turn Cuba "into a f****** parking lot" when given the go-ahead, the Cubans again clamored for inclusion into the Warsaw Pact. Apparently ol' Raul--head of the MINFAR--was told point-blank by the Soviets that they were too far away and that realistically, should the civil wars in Central America become a regional conflagration, there was nothing the Soviets could or indeed, would do. Cuba's response was basically to create the so-called "Guerra de todo el Pueblo" doctrine: e.g. slavishly copy everything the Vietnamese PAVN did and create so-called "mass organization" including a militia and essentially declare a levee-en-masse should the need arise...
Interestingly, the post-USSR Russians maintained the signals intelligence station at Lourdes in Havana Province well into the 21st century. And the Russian navy, such as it is, has visited both Cuban and Venezuelan ports in recent years.